Add frame type for attached modal dialogs

Add a new frame type META_FRAME_TYPE_ATTACHED which is used for
attached modal dialogs.

The theme format version is bumped to 3.2, and attached windows
can have borders defined in a metacity-theme-3.xml as:

 <window version=">= 3.2" type="attached" style_set="[name]"/>

If no style is defined for "attached", drawing will fall back
to the "border" type.

https://bugzilla.gnome.org/show_bug.cgi?id=592382