'Read-only' / Reviewer mode

[BlackFoundry]

It will soon become necessary to have users with read-only access for reviewers.

They should be able to leave comments and notes directly on a Fontra project. (See #58)

Perhaps also have the ability to change the status field of glyphs to mark them 'validated' or any checking preceding step.

[justvanrossum]

Also relates to #58

[justvanrossum]

Let's separate the read-only mode from comments/annotations. The former is being implemented and is fairly simple, but a proper review system with comments / annotation is a serious project that requires design preparations and planning. Let's use issue #58 for that.

[justvanrossum]

For now, we want three modes:

• full editing capabilities
• read-only, selection is possible, but attempting to edit does not work (perhaps shows a dialog)
• "dummy editing": editing is possible, but changes are not written to the backend (this is the current behavior when for example opening a TTF). This is nice to have so someone can play around, get familiar, yet can't destroy the underlying data.