WindowServer: Don't start a drag and drop unless holding Primary mouse

Adds a member to record the last processed mouse buttons. If they
do not include MouseButton::Primary, return early before creating
a new drag and drop client. This fixes race conditions in which
MouseUp events canceling or completing a drop could be swallowed
by Overlay creation or postponed by an executing DragOperation,
leaving the operation in limbo.

Userland/Services/WindowServer/WindowManager.cpp

@@ -1459,6 +1459,7 @@ void WindowManager::event(Core::Event& event)
             m_previous_event_was_super_keydown = false;
 
         process_mouse_event(mouse_event);
+        m_last_processed_buttons = mouse_event.buttons();
         // TODO: handle transitioning between two stacks
         set_hovered_window(current_window_stack().window_at(mouse_event.position(), WindowStack::IncludeWindowFrame::No));
         return;