Recent files menu entry implementation.

[AStepaniuk]

Hi, Here is a PR for #40 .

Some implementation notes:

• Separate class RecentFilesMenu is introduced, which manages "Recent files" menu entry
• The list of recent files is kept persistent in the file "recent_files.list". It is located in the in the same folder where the "remarkable.settings" is
• The maximum files number in recent files list: 10. This can be easily adjusted via code
• File is added to the recent files list on file "Open" and file "Save As" operations (though menu entry is not updated)