[LocationCompleter] Support Ctrl / Shift activation of items

Ctrl activation: Opens url in new tab
Shift activation: Opens url in new window
Closes #1245

Bookmarks: Update visit count also from location completer
BookmarkItem: Added new method updateVisitCount
[BookmarksSidebar] Enable only SingleSelection in sidebar
[CodeCleanup] Use QMouseEvent::button() in mouse release events

In press/move events, use QMouseEvent::buttons() instead