Windows installer: add 3 new features/bugfixes

As discussed with Vincent I added 3 new features:

* LyX can now also be installed over an existing installation of the same LyX version if the users really wants this.
(A message box appears recommending not to do so for test releases and when there are problems with an existing installation of the same release.)
* LyX can now also be installed if the uninstaller executable of a former LyX was accidentally deleted.
(That this case was not handled was more a bug. Thanks Vicent for the pointer.)
* If a user set MiKTeX's automatic package installation to "No" the installer will.
(As discussed with Vincent, if a user has disabled this MiKTeX feature he most probably know the consequences. So respect his decision.)
  respect that and not re-enable it.

I would be happy if anybody can spend a translation of the changed installer message.