switcherPopup: Fix spacing calculation for empty lists

Without special-casing, our current spacing calculation results in
negative size requests for empty lists, which will trigger a Clutter
assert later.
While the list is never supposed to be empty, bugs happen; crashing
users' systems is the least graceful way of handling this, so don't
do it.

https://bugzilla.gnome.org/show_bug.cgi?id=722434
window-tracker: Always enable transient_for redirection

It is possible to associate an application's window with a different
application using the transient_for hint. However we currently only
consider the hint in get_window_app() and not when making the original
association, which opens the door to some confusing inconsistencies;
for instance, get_window_app() will not necessarily return the same
value for all windows retrieved via shell_app_get_windows().
Fix this by looking at the transient_for hint when making the original
association, not just in get_window_app().

https://bugzilla.gnome.org/show_bug.cgi?id=722434
altTab: Always filter out items with no windows

When restricting the switcher popup to the current workspace, we
filter out running apps with an empty window list (namely: no open
windows on the current workspace). However we may end up with an
empty window list even when not restricting items to the current
workspace when all windows of a running app are associated with a
different application via the transient_for hint.
To fix this, just filter out items with an empty window list
unconditionally.

https://bugzilla.gnome.org/show_bug.cgi?id=722434