Start refactoring the windowing system to use an event loop.

Userspace programs can now open /dev/gui_events and read a stream of GUI_Event
structs one at a time.

I was stuck on a stupid problem where we'd reenter Scheduler::yield() due to
having one of the has_data_available_for_reading() implementations using locks.

Widgets/Size.h

@@ -1,9 +1,12 @@
 #pragma once
 
+struct GUI_Size;
+
 class Size {
 public:
     Size() { }
     Size(int w, int h) : m_width(w), m_height(h) { }
+    Size(const GUI_Size&);
 
     bool is_empty() const { return !m_width || !m_height; }
 
@@ -19,6 +22,8 @@ public:
                m_height == other.m_height;
     }
 
+    operator GUI_Size() const;
+
 private:
     int m_width { 0 };
     int m_height { 0 };