- affects the preview video shown during a call.
- moves the preview.
- resizes (keeping the ratio) the preview. Also four zones are used to
anchor the preview when using the maximize/normal button of the window.
- preview is constrained to be fully visible in the videowidget.

Change-Id: Ic7be322a20dbaa4bf455a51eeb9eaedf27ecc5e4
Tuleap: #428