configure_input: Hook up the vibration percentage spinbox

This allows setting the vibration strength percentage anywhere from 1% to 100%.
Also hooks up the remaining motion button and checkbox in the Controller Applet.

src/core/settings.h

@@ -170,6 +170,7 @@ struct Values {
     Setting<bool> use_docked_mode;
 
     Setting<bool> vibration_enabled;
+    Setting<int> vibration_strength;
 
     Setting<bool> motion_enabled;
     std::string motion_device;