Allow triggering custom actions via double-clicking a file in the file view

FossilOrigin-Name: 4270974c3f9f7fb166a74a9d76c9db8a24428fe5

src/Settings.h

@@ -25,6 +25,7 @@ enum FileDblClickAction
 	FILE_DLBCLICK_ACTION_DIFF,
 	FILE_DLBCLICK_ACTION_OPEN,
 	FILE_DLBCLICK_ACTION_OPENCONTAINING,
+	FILE_DLBCLICK_ACTION_CUSTOM,	// Custom Action 1
 	FILE_DLBCLICK_ACTION_MAX
 };