Warn that Extension Manager doesn't work on Linux yet

[pietroalbini]

When I try to install an extension Brackets return an unknow error, even if I use the available section.
Also, if I try to open extensions dir program crash.

Latest version - Ubuntu 12.04

[peterflynn]

Duplicate of #4738 -- installing extensions via the Extension Manager doesn't yet work on Linux. It's waiting on getting Node working, which is on the backlog.

@jasonsanjose Since this seems to be creating a bunch of confusion, should we disable the Extension Manager UI or put in a warning message on Linux until it's working? Leaving this bug open to track that idea for now...

[jasonsanjose]

Until Node.js is implemented, we can just show a warning dialog that the Extension Manager is not supported yet. Tagging starter bug, low priority.

[jasonsanjose]

It turns out that we might not need this warning after all. Node.js integration will likely land this sprint. See adobe/brackets-shell#306 for the branch and a test build.

[jasonsanjose]

Closing. Test build here https://github.com/adobe/brackets/releases/tag/0.30.0-8833.