added popup for handling SIGSEGV or SIGFPE

the popup is placed on each of the virtual screens
the user can decide to restart or quit i3
in case of an exit a core-dump is generated

src/mainx.c

@@ -49,6 +49,7 @@
 #include "manage.h"
 #include "ipc.h"
 #include "log.h"
+#include "sighandler.h"
 
 xcb_connection_t *global_conn;
 
@@ -499,6 +500,7 @@ int main(int argc, char *argv[], char *env[]) {
         /* Handle the events which arrived until now */
         xcb_check_cb(NULL, NULL, 0);
 
+        setup_signal_handler();
         /* Ungrab the server to receive events and enter libev’s eventloop */
         xcb_ungrab_server(conn);
         ev_loop(loop, 0);