Customize shortcut modifier

Add --shortcut-mod, and use Alt as default modifier.

This paves the way to forward the Ctrl key to the device.

app/src/cli.h

@@ -18,4 +18,9 @@ scrcpy_print_usage(const char *arg0);
 bool
 scrcpy_parse_args(struct scrcpy_cli_args *args, int argc, char *argv[]);
 
+#ifdef SC_TEST
+bool
+sc_parse_shortcut_mods(const char *s, struct sc_shortcut_mods *mods);
+#endif
+
 #endif