media library: fix path history for items with a real path differing from the requested path

This fixes a problem in media library navigation brought to my attention by @mkortstiege as described in http://forum.kodi.tv/showthread.php?tid=211165. Basically the problem is that when using a `library://` based URL to access media library items from the home screen the `return` statement in the `ActivateWindow()` builtin doesn't have the expected effect.

The reason for this is that `CGUIMediaWindow` is opened with a `library://` based URL but that URL is changed to a `videodb://` based URL during `GetDirectory` which is required for proper filtering etc. Due to the change of the URL during `GetDirectory` the navigation history in `CGUIMediaWindow` is wrong and still contains `library://` based URLs instead of `videodb://` based URL.

There is already some handling for this case for `CGUIMediaWindow::m_startDirectory` but it doesn't cover the navigation history yet which is added by this commit.

This approach is not a 100% clean solution but I'm not very familiar with the navigation history stuff (@jmarshallnz is probably the only one that was) and this is the best I could come up with. The problem I have with changing things in `CGUIMediaWindow` is that it's impossible to test all use cases to be sure that there are no regressions.
Fix is correct from what I can tell, though I don't believe you need to call `ClearPathHistory()` as this is done in `SetHistoryForPath` by the looks (and is not called in other cases we do this).