Fix media stubs

When playing media stubs, the "Please insert disc" dialog wasn't showing.

In the **getPlayUrl()** function, the playurl was changing according to the **VideoType**. But if the file is a media stub, we don't want that. We will just use the **path** and XBMC will handle it.
So, we will check the **VideoType** only if the file it is not a media stub. For that we check the **IsPlaceHolder** property, which had a typo (not a capital H), and was comparing to the string "true" instead of the boolean **True** (this was happening in the **default.py** file).

So, because we WANT to play the media stubs, I remove the check for **IsPlaceHolder** from the if statement of the **Can not play virtual items** block. I did this only in the **PLAY** method. This happens too in the **PLAYPlaylist** one, but I don't know exactly what it does, so I just corrected the typo.

This makes the **PLAY** method show the correct dialog, and **PLAYPlaylist** to show the "This item is not playable" dialog.