subtree(3p/exwm): update to commit 'b62d5e79'

This is almost one year of changes to EXWM. Note that it undoes our
port of ch11ng/exwm#737

That PR hasn't seen any movement in three years, so it might not be
that relevant anymore. Other stuff has been mainlined in the meantime.

Change-Id: I0845ff8a28a5bb1553855f6d6f0ceeaedcf0809e