PixelPaint: Refactor main.cpp into MainWidget

Previously, all the UI setup was done in `main.cpp`, with a whole
bunch of lambdas,, and actions etc just being stored in the main
function. This is probably an artifact from back when it was first
created.

Most other applications now have a "MainWidget" class of some sort
which handles setting up all the UI/menubars, etc. More importantly,,
it also lets us handle application-wide events which we were
previously not able to do directly, since the main widget was just
a default GUI::Widget.

This patch moves all the core functionality of the PixelPaint
application into PixelPaint::MainWidget, which is then instantiated
by the main function. There is likely some more refactoring that
would help, but this commit is big enough as it is doing mostly
a direct port.

Userland/Applications/PixelPaint/CMakeLists.txt

@@ -24,6 +24,7 @@ set(SOURCES
     LayerListWidget.cpp
     LayerPropertiesWidget.cpp
     LineTool.cpp
+    MainWidget.cpp
     main.cpp
     MoveTool.cpp
     PaletteWidget.cpp