Display current project name in main window

Supersedes #6878.

This implements #2281, displaying the project name in the Brackets window title.
This feels a bit hacky, and it won't work if there's a dash in the filename.  How about using a structure like this?

```
var projectRoot = ProjectManager.getProjectRoot();
if (projectRoot) {
    if (currentlyViewedPath) {
        windowTitle = ...;
        if (currentDoc && currentDoc.isDirty) {
            windowTitle = "• " + windowTitle;
        } // (note: we can lose the 'else' case with _$dirtydot in the original code - it's unneeded)
    } else {
        windowTitle = ...;
    }
} else {
    windowTitle = brackets.config.app_title;
}
```

This centralizes all the cases so it's easier to see what they are, and it more cleanly aligns with the code above that updates the HTML menu/titlebar.