Refactor TitleBar updates [Was: Initial fix for issue #95]

Since this is becoming more than just a simple update to stop the current broken behavior, I've renamed it.

The initial commit seems to work for the simple cases
* EnableTitle = $true (default), Titlebar = default
* EnableTitle = $false, TitleBar = default
* EnableTitle = $true, TitleBar = default (current default behavior)
* EnableTitle = string, TitleBar set by user

Later updates will separate out the titlebar logic into a Set-VcsTitle method following suggested best practices.