always open workspace external files read only

[nittka]

This PR ensures that workspace external files are opened read only. As a consequence https://github.com/thSoft/eclipse-commons/pull/43 will not cause problems.

[thSoft]

Hmm, on second thought, we do want to support editing non-workspace files, don't we?

[nittka]

Well, maybe. But for now, this is the safer option (and consisent with the previous behaviour).

There are some problems with writable workspace external files:

• you could navigate to default imports, edit and save them, messing up the lilypond installation
• workspace external files are not part of the build, i.e. they will not cause including files to be marked as dirty

My current argument is that if some sources you want to edit are not already in some workspace project, you can use import file system (import as a linked folder) in order to get them into the workspace.