Users should be able to mark a note as important

[Stefano Verzegnassi]

Quick Memo needs a way to manage notes by their importance.

A possible solution:
1) Add an Action in MemoPage's Header, so user can mark the note as important.
2) Provided a visual feedback through the delegates used in Overview (a.k.a. MainPage), so user can recognise starred notes from not-starred ones.
3) Add a "Starred" page, where only starred notes are shown. This page should be reachable from Header Drawer.
4) (Optional) Let the user mark multiple notes, through MultiSelectionHandler.

NoteModel would require some changes, retro-compatibility with older notes should be kept.