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

Checking for GTK2 theme engine is unnecessary, as the install just
copies the files. If GTK2 is not available, the theme will just not be
present.

A dark variant of BlueMenta with blue accent elements.

This theme used to be called Mist, later renamed to Fog, and eventually
removed. This patch brings it back with some updated components. It does
not provide a full desktop theme (i.e. no colours or gtk widget
controls), but just the window decorations and icon theme.

Some distros provide a theme called Mist that only provides a gtkrc
file, and so GTK widgets will just default to Adwaita.

This theme used to be called Crux, later renamed to Quid, and eventually
removed. This patch brings it back with some updated components. It does
not provide a full desktop theme (i.e. no colours or gtk widget
controls), but just the window decorations and icon theme.

Some distros provide a theme called Crux that only provides a gtkrc
file, and so GTK widgets will just default to Adwaita.