Added (GCallback) cast to silence warnings

src/mainwindow.c

@@ -175,15 +175,15 @@ static void mainWindow_connect_signals(struct mainWindow *mainWindow)
 {
 	g_signal_connect(mainWindow->dateStart,
 			 "focus-in-event",
-			 mainWindow_showPopover,
+			 (GCallback) mainWindow_showPopover,
 			 mainWindow);
 	g_signal_connect(mainWindow->dateEnd,
 			 "focus-in-event",
-			 mainWindow_showPopover,
+			 (GCallback) mainWindow_showPopover,
 			 mainWindow);
 	g_signal_connect(mainWindow->calendarRange,
 			 "day-selected",
-			 mainWindow_calendarSelected,
+			 (GCallback) mainWindow_calendarSelected,
 			 mainWindow);
 }