WindowServer: Make various functions take MouseEvent by const reference

Some paths of the mouse event processing code will upgrade the event
from a regular MouseDown to a MouseDoubleClick. That's why we were
passing `MouseEvent&` everywhere.

For the paths that don't need to do this, passing `MouseEvent const&`
reduces the cognitive burden a bit, so let's do that.

Userland/Services/WindowServer/WindowManager.cpp

@@ -896,7 +896,7 @@ void WindowManager::process_event_for_doubleclick(Window& window, MouseEvent& ev
     metadata.last_position = event.position();
 }
 
-void WindowManager::deliver_mouse_event(Window& window, MouseEvent& event, bool process_double_click)
+void WindowManager::deliver_mouse_event(Window& window, MouseEvent const& event, bool process_double_click)
 {
     auto translated_event = event.translated(-window.position());
     window.dispatch_event(translated_event);
@@ -907,7 +907,7 @@ void WindowManager::deliver_mouse_event(Window& window, MouseEvent& event, bool
     }
 }
 
-bool WindowManager::process_ongoing_active_input_mouse_event(MouseEvent& event)
+bool WindowManager::process_ongoing_active_input_mouse_event(MouseEvent const& event)
 {
     if (!m_active_input_tracking_window)
         return false;
@@ -927,7 +927,7 @@ bool WindowManager::process_ongoing_active_input_mouse_event(MouseEvent& event)
     return true;
 }
 
-bool WindowManager::process_mouse_event_for_titlebar_buttons(MouseEvent& event)
+bool WindowManager::process_mouse_event_for_titlebar_buttons(MouseEvent const& event)
 {
     if (m_cursor_tracking_button) {
         m_cursor_tracking_button->on_mouse_event(event.translated(-m_cursor_tracking_button->screen_rect().location()));
@@ -941,7 +941,7 @@ bool WindowManager::process_mouse_event_for_titlebar_buttons(MouseEvent& event)
     return false;
 }
 
-void WindowManager::process_mouse_event_for_window(HitTestResult& result, MouseEvent& event)
+void WindowManager::process_mouse_event_for_window(HitTestResult& result, MouseEvent const& event)
 {
     auto& window = *result.window;
 

Userland/Services/WindowServer/WindowManager.h

@@ -182,7 +182,7 @@ public:
     void invalidate_after_theme_or_font_change();
 
     bool set_hovered_window(Window*);
-    void deliver_mouse_event(Window&, MouseEvent&, bool process_double_click);
+    void deliver_mouse_event(Window&, MouseEvent const&, bool process_double_click);
 
     void did_popup_a_menu(Badge<Menu>);
 
@@ -239,9 +239,9 @@ private:
     bool process_ongoing_window_resize(MouseEvent const&);
     bool process_ongoing_window_move(MouseEvent&);
     bool process_ongoing_drag(MouseEvent&);
-    bool process_ongoing_active_input_mouse_event(MouseEvent&);
-    bool process_mouse_event_for_titlebar_buttons(MouseEvent&);
-    void process_mouse_event_for_window(HitTestResult&, MouseEvent&);
+    bool process_ongoing_active_input_mouse_event(MouseEvent const&);
+    bool process_mouse_event_for_titlebar_buttons(MouseEvent const&);
+    void process_mouse_event_for_window(HitTestResult&, MouseEvent const&);
 
     template<typename Callback>
     void for_each_window_manager(Callback);