preview-mode: Add runtime UI

This pull request adds the UI infrastructure in Frescobaldi to select preview modes at runtime.
I think it _contains_ the other pull request.

I think this is quite clean with regards to the infrastructure.

What now has to be done is to clean up the include files with the actual modes and to add more modes.

A second major step will be to add infrastructure and UI to configure the preview modes.
Maybe this can be in the panel, or in the preferences dialog.
I'm thinking of options like which colors to use, line thickness or reference point symbol etc.