Better accesibility to maximize view

Summary:
- Added handlig of mouse double click on view header bar to toggle
  maximize.
- Added menu entry under View -> Split view, Toogle maximize view

Test plan:
A.1. Open Konsole and create split views
A.2. Double clicking a view header should toggle maximize/restore the view
B.1. Open Konsole and create split views
B.2. Go to menu View -> Split view -> Toggle maximize view