Everywhere: Use 'decrease_slider_by()' method from AbstractSlider

The same idea as 'increase_slider_by()', it helps us to avoid repeating
the pattern 'set_value(value() - delta)'.

Userland/Applications/PDFViewer/PDFViewer.cpp

@@ -106,7 +106,7 @@ void PDFViewer::mousewheel_event(GUI::MouseEvent& event)
                     scrollbar.set_value(scrollbar.max());
                 }
             } else {
-                scrollbar.set_value(scrollbar.value() - 20);
+                scrollbar.decrease_slider_by(20);
             }
         }
     }

Userland/Applications/PixelPaint/Tools/Tool.cpp

@@ -34,7 +34,7 @@ void Tool::on_keydown(GUI::KeyEvent& event)
     switch (event.key()) {
     case KeyCode::Key_LeftBracket:
         if (m_primary_slider)
-            m_primary_slider->set_value(m_primary_slider->value() - 1);
+            m_primary_slider->decrease_slider_by(1);
         break;
     case KeyCode::Key_RightBracket:
         if (m_primary_slider)
@@ -42,7 +42,7 @@ void Tool::on_keydown(GUI::KeyEvent& event)
         break;
     case KeyCode::Key_LeftBrace:
         if (m_secondary_slider)
-            m_secondary_slider->set_value(m_secondary_slider->value() - 1);
+            m_secondary_slider->decrease_slider_by(1);
         break;
     case KeyCode::Key_RightBrace:
         if (m_secondary_slider)

Userland/Libraries/LibGUI/OpacitySlider.cpp

@@ -139,7 +139,7 @@ void OpacitySlider::mouseup_event(MouseEvent& event)
 
 void OpacitySlider::mousewheel_event(MouseEvent& event)
 {
-    set_value(value() - event.wheel_delta());
+    decrease_slider_by(event.wheel_delta());
 }
 
 }

Userland/Libraries/LibGUI/Scrollbar.cpp

@@ -329,7 +329,7 @@ void Scrollbar::scroll_by_page(const Gfx::IntPoint& click_position)
 
     if (click_position.primary_offset_for_orientation(orientation()) < scrubber_rect().primary_offset_for_orientation(orientation())) {
         gutter_click_state = GutterClickState::BeforeScrubber;
-        set_value(value() - page_increment);
+        decrease_slider_by(page_increment);
     } else {
         gutter_click_state = GutterClickState::AfterScrubber;
         increase_slider_by(page_increment);

Userland/Libraries/LibGUI/Slider.cpp

@@ -147,7 +147,7 @@ void Slider::mousewheel_event(MouseEvent& event)
         wheel_delta /= abs(wheel_delta);
 
     if (orientation() == Orientation::Horizontal)
-        set_value(value() - wheel_delta * acceleration_modifier);
+        decrease_slider_by(wheel_delta * acceleration_modifier);
     else
         increase_slider_by(wheel_delta * acceleration_modifier);
 

Userland/Libraries/LibGUI/ValueSlider.cpp

@@ -52,7 +52,7 @@ ValueSlider::ValueSlider(Gfx::Orientation orientation, String suffix)
 
     m_textbox->on_down_pressed = [&]() {
         if (value() > min())
-            AbstractSlider::set_value(value() - 1);
+            AbstractSlider::decrease_slider_by(1);
         m_textbox->set_text(formatted_value());
     };
 
@@ -161,7 +161,7 @@ void ValueSlider::mousewheel_event(MouseEvent& event)
     if (event.wheel_delta() < 0)
         increase_slider_by(1);
     else
-        set_value(value() - 1);
+        decrease_slider_by(1);
 }
 
 void ValueSlider::mousemove_event(MouseEvent& event)

Userland/Libraries/LibVT/TerminalWidget.cpp

@@ -215,7 +215,7 @@ void TerminalWidget::keydown_event(GUI::KeyEvent& event)
     m_cursor_blink_timer->start();
 
     if (event.key() == KeyCode::Key_PageUp && event.modifiers() == Mod_Shift) {
-        m_scrollbar->set_value(m_scrollbar->value() - m_terminal.rows());
+        m_scrollbar->decrease_slider_by(m_terminal.rows());
         return;
     }
     if (event.key() == KeyCode::Key_PageDown && event.modifiers() == Mod_Shift) {

Userland/Libraries/LibWeb/InProcessWebView.cpp

@@ -269,7 +269,7 @@ void InProcessWebView::keydown_event(GUI::KeyEvent& event)
             vertical_scrollbar().increase_slider_by(frame_inner_rect().height());
             break;
         case Key_PageUp:
-            vertical_scrollbar().set_value(vertical_scrollbar().value() - frame_inner_rect().height());
+            vertical_scrollbar().decrease_slider_by(frame_inner_rect().height());
             break;
         default:
             if (!page_accepted_event) {