Add a configure flag to optionally skip installation of GTK2 theme
files. Enabled by default for backward compatibility. Distros that have
dropped GTK2 can pass --disable-gtk2 to avoid installing unused theme
files.

Fixes #332

The xx-small scale makes title bars really small and harder to read, which
is not really the goal of high contrast themes :)

This completes cf0a970 ("Fix HighContrast themes visibility with
metacity").

When maximized windows are rendered with metacity (e.g. with
metacity-theme-viewer), the back background parts of HighContrast and
HighContrastInverse actually show up transparent. This is because the
maximized_unfocused and maximized_focused styles do not use the proper
focus draw_ops like normal_focused and normal_unfocused.

When rendered with metacity (e.g. with metacity-theme-viewer), the
back background parts of HighContrast and HighContrastInverse actually
show up transparent. This is because the corresponding rectangles were
missing the filled attribute.

In the HighContrast case, the gtk_arrow is getting drawn black on black (and
there is currently no way to change the color), so we can as well draw
it by hand to be able to change the color.

The close button also deserves bigger width to be more visible.

Fixes #211

This reverts commit 406802bc6f8939415ffc51a429aab948ce216571.

The name change prevent us to do any change in our theme,
it seems that gtk+ version will always be prefered if we use the same name.