Linux native menus

[eyelash]

I improved the Linux native menus a bit. There is still a lot to do (and I will keep working on it) but the improvements are already worth merging.

[ingorichter]

@eyelash Thanks for getting this implemented. A couple of months ago (sigh), I proposed to keep a feature branch to implement this feature. This way we can implement and test everything in isolation before merging to master without having to fear any issues in master.

[eyelash]

You could still create such a feature branch and then merge my branch into the feature branch instead of master.
Originally I wanted to work on your branch but then pull request #348 was merged into master so I thought it was OK to work on the master branch.

[ingorichter]

That what I'm going to do.

[ingorichter]

@eyelash I have created the feature branch in https://github.com/adobe/brackets-shell/tree/ingo/linux-native-menus and merged your changes into this branch already. I'm going to close this PR in favor of the new branch. Thanks!