PixelPaint: Make the main UI tabbed and allow multiple open images :^)

This patch adds a GUI::TabWidget to the main UI and allows having
multiple images open at the same time.

Some of the changes here are a bit hackish and mechanical and there's
still code around that needs more work to fit better in the new world.

One nice side-effect of this change is that ImageEditor now always
has one Image associated with it, and it never changes.

Userland/Applications/PixelPaint/Tool.h

@@ -31,6 +31,9 @@ public:
     void clear() { m_editor = nullptr; }
     void setup(ImageEditor&);
 
+    ImageEditor const* editor() const { return m_editor; }
+    ImageEditor* editor() { return m_editor; }
+
     GUI::Action* action() { return m_action; }
     void set_action(GUI::Action*);