Remove notes on number entry

[jlnordin]

This PR adds a feature to automatically remove all the notes in the same column, row, and box when entering a number in a cell. This can be disabled in settings and is off by default. This addresses feature request #31 .

If you use this feature, both the number entry and all the note removals are grouped together into the same command so that you can undo the entire operation as a logical unit.

This PR also adds a new abstract base class -- AbstractMultiNoteCommand -- to centralize some of the common behavior between FillInNotesCommand, ClearAllNotesCommand, and the new SetCellValueAndRemoveNotesCommand.

Scenarios Tested

• Verified entering a number with and without the remove notes feature active.
• Verified clearing an existing number doesn't cause a crash (when trying to remove notes for '0').

[ogarcia]

Sorry for delay, along this week I'll merge and upload new version :wink: