QmlDesigner.ItemLibrary: various updates

- clearing search input focus when clicking clear icon
- re-set updated sections' models
- using GridView-based item selection
- C++ code cleanup
- Manhattan-length used for drag & drop initiation
- integrated QtCreator's filter text input
- replaced QToolButtons with a QTabBar
- fix: eliding text & corrected selector offset
- styled resources view
- updated Qml syntax