Improve UI for Bookmarks sidebar page buttons

Currently there are two tiny icons-only buttons with very similar icons
(which may even be identical in some icon themes). One of them is
checkable, and the other one triggers an immediate action. And there is
lots of unused space to the right of them.

This commit overhauls them in the following way:
- The toggle button becomes a checkbox with visible text, and is
  inverted; now when checked, it shows all bookmarks
- The checkbox moves above the view and search field, to communicate
  that it affects everything below it
- The button to add a bookmark gets some visible text

This should make the UI much clearer, at the cost of one extra line of
vertical space from the bookmarks list.