[win32/nsis/rebrand] - don't force remove former XBMC installation

As proposed in the forum - this is the RFC pull request.

1. Don't force remove old XBMC installations when installing kodi
2. Only migrate the XBMC data folder to Kodi when no Kodi folder is found already (elsethe nagging MessageBox which tells the user that data folder was moved would come in error)
3. Make the if constructs a bit more readable by using nsis conditions without relative offsets (use EndIf instead).

Err, "a previous *XBMC*" installation? What about previous Kodi installations?