main: move pointer-sync-on-restack code here

We call global.sync_pointer() on MetaScreen::restack as a hack to try
to fix up the hover state after a pointer grab. Previously we were
doing this in chrome.js, since there was already a ::restack handler
there anyway, but this isn't really related to the chrome at all, so
move it to main.js instead.

https://bugzilla.gnome.org/show_bug.cgi?id=633620