Add native menus switch

The linux branch uses a native file system and HTML menus, so the logic around `brackets.inBrowser` is getting a bit complicated. This change adds a new `brackets.nativeMenus` flag to simplify the logic.

Note, the real reason I added this was to be able to quickly test native menus on any platform by simply settting:

```
    global.brackets.nativeMenus = false;
```

@jasonsanjose Please review Linux logic.

/cc @peterflynn