LibGUI+WindowServer: Implement drag-to-select behavior in GTextEditor.

To make this feel right, I needed to start passing keyboard modifiers along
with mouse events. That allows shift-clicking to extend the selection. :^)

WindowServer/WSScreen.h

@@ -38,6 +38,7 @@ private:
 
     Point m_cursor_location;
     unsigned m_mouse_button_state { 0 };
+    unsigned m_modifiers { 0 };
 };
 
 inline RGBA32* WSScreen::scanline(int y)