altTab: make thumbnails scroll to right if needed at first Alt+Shift+Above_Tab

altTab: make thumbnails scroll to right if needed at first Alt+Shift+Above_Tab

Force an allocation at thumbnails creation so we can figure out whether we
need to scroll when selecting.

We also need to show() the whole AltTabPopup before calling _select() so that,
when computing the scrolling offset, the widgets already have their styles
loaded. Otherwise we will miss the switcher list item container's spacing.

https://bugzilla.gnome.org/show_bug.cgi?id=655069