Adding Qt5 support for qterminal

To build x11-terms/qterminal with Qt5 x11-libs/qtermwidget also needs to be built with Qt5. The 'qtnext' USE flag used here to enable Qt5 in these ebuilds can be replaced by the appropriate 'qt5' USE flag easily, when the latter is unmasked officially.
You can use here `cmake-utils_use_use` to also deactivate Qt5 with `-DUSE_QT5=OFF`.