window: Treat CurrentTime as legal timestamp in activation

Effectively we have been accepting CurrentTime timestamps for years,
but still complained about "stupid pagers" when encountering them;
just accept that we will never limit treating 0 timestamps as current
time to pagers.

https://bugzilla.gnome.org/show_bug.cgi?id=728018
window: Allow activation on non-active workspaces with proper timestamps

Our focus stealing prevention is still mostly inherited from metacity;
in particular, a (non-transient) window that is not on the current
workspace will not be given focus. This behavior made sense in the
GNOME 2 days, where workspaces were separated much more strictly.
However this is no longer the case in GNOME 3 - activating a launcher
will switch workspaces if necessary, and so will the app switcher.
There is no good reason to not do the same for other user actions
like clicking a URL or activating a search result, so allow activation
of windows on non-active workspaces if a proper timestamp is supplied,
assuming that this is a strong enough indication that we are dealing
with a legitimate user action.

https://bugzilla.gnome.org/show_bug.cgi?id=728018