TabSheet focus/blur API improvement (#14304)

Simplify the new inner API in the VTabSheet which solves the
focus/blur issue.

The blur and focus events should be linked with the TabSheet
component as a whole. Any click inside the TabSheet should
trigger one single focus and any leave the blur.

Change-Id: I90dd05f056441920a286b6debdc288fb1a9a7e10