[poppler] [RFC] Drop the qt4 frontend at the end of the year

[Albert Astals Cid]

Hi, as mentioned partially in another thread I'd like to drop the qt4 frontend 
at the end of the year.

Reasons:
 * Qt upstream stopped supporting Qt4 in May 2015
 * KDE (one of the major Qt users) will stop releasing apps based in Qt4 at 
the end of the year too.


I would not mind keeping the code if someone else steps up to maintain it, but 
*personally* I will not care about it, or have qt4 installed to check if 
things even compile after the end of the year.

Cheers,
  Albert