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/PixelPaintWindow.gml

@@ -32,8 +32,8 @@
                 spacing: 0
             }
 
-            @PixelPaint::ImageEditor {
-                name: "image_editor"
+            @GUI::TabWidget {
+                name: "tab_widget"
             }
 
             @PixelPaint::PaletteWidget {