Change ToolBar accordingly to the contents

When we are showing the preflets grid,toolbar only shows a search box,
but when we are shoing a preflet the search box disappears, a back
action and the preflet title are shown instead.

Issue: #13
Hide the search box for now

Don't show it until we have a searchable categorized grid view.

Issue: #13
Custom style for group boxes

Make sure group boxes are styled the way we want despite the current
QtQuick Controls style.

Issue: #13