Allow language to be marked as readonly

[DaveGreasley]

It would be useful if, rather than removing a language altogether (using the FilterLanguages event), we could mark a language as readonly so that the value can be seen but not updated.

[mattbrailsford]

Just adding a comment to this really. This is not as simple as it first appears. The fix in your repo is fine for anything that is an input field, but people put many types of property editors inside vorto including things like the grid which is pretty dificult to make read only. My concern would be we implement this and people could modify something even if it's flagged to be readonly.