Fix page header click events propagating into main ScrollView

The page header previously ignored click events, so these were sent to
the underlying ScrollView. qqc2-desktop-style would clip ScrollView,
which is why this wasn't noticed earlier.

To solve this I have ported HeaderFooterToolbar to a QQC2.ToolBar, which
handles focus/mouse clicks for us. This is probably more efficient than
clipping the ScrollView.

Test-plan:
- Open albums view
- Set view-mode to grid
- Scroll the page down slightly, so that an album image is underneath
  the header toolbar
- Click in an empty space in the toolbar

Before:
- The click is accept by the ScrollView, and an album page opens

Now:
- The click is intercepted by the toolbar and nothing happens


(cherry picked from commit cc1fa5fb)