[x11] Ensure that mostRecentlyActivatedClient() returns correct values
If a FocusIn event does not originate from the window manager and our focus stealing prevention policies are fine with granting focus to a client that has triggered the FocusIn event, we still need to clean up the should_get_focus list; otherwise mostRecentlyActivatedClient() may return a client that is not actually the most recently activated one.
Edited by Vlad Zahorodnii