If we're sharing buffers, we only want to share trivial structures
as anything else could potentially share internal pointers, which
most likely is going to cause problems due to different address
spaces.
Fix the GUI::SystemTheme structure, which was not trivial, which
is now caught at compile time.
Fixes#3650
Change #2811 made window title stripes and window title shadow themable,
but it used the same stripe and shadow color for all window modes.
This is fine for the new 'basalt' theme which uses the same color
in all four window modes, but it changed the default theme so that
background windows had brown stripes and a brown shadow.
Instead, make the title stripe and title shadow themable per window mode,
and change the default theme to restore the colors it had before
change #2811: The title stripe color is the same as Border1 for all
window modes, and the title shadow is the same as the title stripe
darkened by 0.6.
The theming system can now control title bar height, title button
size, title stripe color and the title text shadow color.
The implemented theme metrics system could be later extended to LibGUI
to allow themes to change widget padding, border width, etc.
