Remove weird SigListener class

The SigListener class was used to queue folders that we need to sync and
to receive "resync" and "abort" signals. It was undocumented and weird
and we had to pass "siglisteners" through the whole program.

Simply do away with it, and make 2 functions in the Account() class:
set_abort_event and get_abort_event which can be used to set and check
for such signals. This way we do not need to pass siglisteners all over
the place. Tested Blinkenlights and TTYUI uis to make sure that SIGUSR1
and SIGUSR2 actually still work.

Document those signals in MANUAL.rst. They were completly undocumented.

This simplifies the code and interdependencies by passing less stuff
around. Removes an undocumented and weirdly named class.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

docs/MANUAL.rst

@@ -262,6 +262,21 @@ MachineUI generates output in a machine-parsable format.  It is designed
 for other programs that will interface to OfflineIMAP.
 
 
+Signals
+=======
+
+OfflineImap listens to the unix signals SIGUSR1 and SIGUSR2.
+
+If sent a SIGUSR1 it will abort any current (or next future) sleep of all
+accounts that are configured to "autorefresh". In effect, this will trigger a
+full sync of all accounts to be performed as soon as possible.
+
+If sent a SIGUSR2, it will stop "autorefresh mode" for all accounts. That is,
+accounts will abort any current sleep and will exit after a currently running
+synchronization has finished. This signal can be used to gracefully exit out of
+a running offlineimap "daemon".
+
+
 KNOWN BUGS
 ==========