Improved UI: Large buttons are not shown.

* In the Personalization Tab Page, the button sizes are not affected
  by the presence of the preview buttons. THey remain their original
  size.
* Added some padding and spacing to avoid cluttering of the widgets.

Change-Id: Iea919b9fbfce4a7bfc82f73ccb49f94aa10ec804
The "Select personas installed via extensions" label is invisible by default.

The label is not shown if there are no persona extensions installed.

Change-Id: I758bb7ff7f22d56bfb7fe212908f9f5f1f3e7979