System message queue

The screen and user interaction is managed by the main thread. Other parts of
calcurse (threads) wishing to use the screen or communicate with the user, must
do it via the main thread.

For this purpose the main input loop is extended with a message queue. A thread
may insert a message in the queue. The main thread tests for messages before
listening for user commands. If a message is present, it is displayed (in a popup
window) for the user to acknowledge. Depending on the message other actions may
be performed, e.g. the message could be turned into a "system appointment/event"
and inserted among the usual appointments.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/Makefile.am

@@ -29,6 +29,7 @@ calcurse_SOURCES = \
 	note.c \
 	notify.c \
 	pcal.c \
+	queue.c \
 	recur.c \
 	sha1.c \
 	sigs.c \