IconTheme: Move changed emission to an idle

By delaying the emission to an idle we avoid a lot of tricky
reentrancy issues. For instance, a normal gtk_icon_theme_choose_icon()
call could in very rare cases (when a user updated an icon theme) emit
a signal which could affect the icon currently being looked up.  This
kind of reentrancy is very hard to test against, especially when it is
so rare, so we're better of avoiding it.

There is no real value to get the change signal directly anyway. All
it can do is affect which icon is rendered the next frame, and we will
handle the queued emission before rendering. Not to mention that icon
theme change detection is polled anyway, so it is already delayed.

https://bugzilla.gnome.org/show_bug.cgi?id=694755