WindowServer+LibGfx: Move normal window frame painting to a WindowTheme

This patch introduces the ClassicWindowTheme, which is our default
theme implemented as a Gfx::WindowTheme subclass.

In this initial cut, we move normal window frame painting and title
bar metrics helpers out of WindowServer and into LibGfx.

This will eventually allow us much greater flexibility with theming
windows, and also makes it easier to build applications that want to
render a window with a specific style for some reason. :^)

Libraries/LibGfx/CMakeLists.txt

@@ -3,6 +3,7 @@ set(SOURCES
     Bitmap.cpp
     BMPLoader.cpp
     CharacterBitmap.cpp
+    ClassicWindowTheme.cpp
     Color.cpp
     DisjointRectSet.cpp
     Emoji.cpp