Add Alt+Enter button to Utilities page, update Win Alt+Tab func

Requested in this reddit post:
https://www.reddit.com/r/Vive/comments/agnvux/advanced_settings_280_motion_smoothing_audio_and/ee7muqo/

Use case is toggling full screen mode on applications.

The Alt+Tab function has been updated with a constexpr var that reduces
duplication in the function calls and makes the keys even more explicit.

src/tabcontrollers/UtilitiesTabController.cpp

@@ -78,6 +78,11 @@ void UtilitiesTabController::sendKeyboardEnter()
     keyboardInput->sendKeyboardEnter();
 }
 
+void UtilitiesTabController::sendKeyboardAltEnter()
+{
+    keyboardInput->sendKeyboardAltEnter();
+}
+
 void UtilitiesTabController::sendKeyboardAltTab()
 {
     keyboardInput->sendKeyboardAltTab();