Prepare LyX code to compile and link with Qt5:

Prepare LyX code to compile and link with Qt5:
* some functionality is in new modules now
  new header locations and library names: QtConcurrent and QtWidgets
* method setResizeMode is renamed to setSectionResizeMode
* deprecated QAbstractItemModel::reset() is dropped now
* platform specific code like QApplication::syncX() is not common anymore
* QString::fromAscii() is dropped now
* some QDesktopServices methods has been moved to QStandardPaths