Fix not able to close drawer using system back button

[ValentinBuira]

This PR try to fix an error where in some case you couldn't close a drawer using the system back button

This happened because the control focus was not transferred correctly

This bug was found during release testing and relate to #3525

[ValentinBuira]

I also fixed the same issue for MMFormPage

I have been pondering and in the end I moved the handling event to the root of MMFormPage, and I removed this part of the code which as no used anymore AFAIK:

-    onVisibleChanged: function( visible ) {
-      if ( visible )
-        backHandler.forceActiveFocus()
-    }

[tomasMizera]

This is looking good, let's see what testing brings