LibGUI: Show pressed state for Space and Return key events

Also allows the user to press Esc while the button is being pressed
to cancel the button action.

Userland/Libraries/LibGUI/AbstractButton.cpp

@@ -146,13 +146,29 @@ void AbstractButton::leave_event(Core::Event&)
 void AbstractButton::keydown_event(KeyEvent& event)
 {
     if (event.key() == KeyCode::Key_Return || event.key() == KeyCode::Key_Space) {
-        click(event.modifiers());
+        m_being_pressed = true;
+        update();
+        event.accept();
+        return;
+    } else if (m_being_pressed && event.key() == KeyCode::Key_Escape) {
+        m_being_pressed = false;
+        update();
         event.accept();
         return;
     }
     Widget::keydown_event(event);
 }
 
+void AbstractButton::keyup_event(KeyEvent& event)
+{
+    if (m_being_pressed && (event.key() == KeyCode::Key_Return || event.key() == KeyCode::Key_Space)) {
+        click(event.modifiers());
+        event.accept();
+        return;
+    }
+    Widget::keyup_event(event);
+}
+
 void AbstractButton::paint_text(Painter& painter, const Gfx::IntRect& rect, const Gfx::Font& font, Gfx::TextAlignment text_alignment)
 {
     auto clipped_rect = rect.intersected(this->rect());

Userland/Libraries/LibGUI/AbstractButton.h

@@ -46,6 +46,7 @@ protected:
     virtual void mousemove_event(MouseEvent&) override;
     virtual void mouseup_event(MouseEvent&) override;
     virtual void keydown_event(KeyEvent&) override;
+    virtual void keyup_event(KeyEvent&) override;
     virtual void enter_event(Core::Event&) override;
     virtual void leave_event(Core::Event&) override;
     virtual void change_event(Event&) override;