Handle SIGINT through libev

Handle SIGINT using libev's ev_signal handler, instead of using
sigaction().

Fixes #64. But compton might miss signals when it's not in the main loop
(e.g. when compton is reset), and thus not exit "clearly". It should
cause any real problem though, so we don't care.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/common.h

@@ -311,6 +311,8 @@ typedef struct session {
   ev_prepare event_check;
   /// Signal handler for SIGUSR1
   ev_signal usr1_signal;
+  /// Signal handler for SIGINT
+  ev_signal int_signal;
   /// backend data
   void *backend_data;
   /// libev mainloop