Display current project name in main window

Supersedes #6878.

This implements #2281, displaying the project name in the Brackets window title.
It doesn't seem like we need to store this in a var.  We can just read `ProjectManager.getProjectRoot().name` in `_updateTitle()` below.

If we do keep this here, let's leave it uninitialized instead of initializing to `""`, which shouldn't be needed.