theme: Rename button_rect() to get_button_rect()

Basically it's odd to have "button_rect" be a function with all the
foo_rect GdkRectangles around - renaming to get_button_rect() will
free the name for the generically named "rect" once buttons are the
only movable pieces in the frame.

https://bugzilla.gnome.org/show_bug.cgi?id=741917
theme: Add titlebar_spacing

Rather than defining the space to the left and right of buttons, add a
simple spacing property that defines the space between buttons, which is
what GTK+ does for client-side decorations (e.g. GtkButtons in a GtkBox).
Unfortunately the value is hardcoded in GTK+; if it is exposed in the
theme in the future, we should pick it up from there, but for now we
just use the same value as GTK+.

https://bugzilla.gnome.org/show_bug.cgi?id=741917