Adapt to new tomboy-library

This PR adapts tomboy.osx to the changes I did in tomboy-library master.

On a side note and unrelated to this PR, I discovered that indentation & whitespaces is completely broken in tomboy.osx. Tabs & spaces are mixed everywhere, some lines use tabs, some spaces etc. We should decide on a style guide for tomboy-notes organization and put them as project-specific settings into all relevant .csproj files.