ViewConfig: Use new generic config interface

This is a somehow raw transition, autotests are adapted too and still working
but there may still some adjustments needed, like value validators, fix
accidentally wrong converted default settings or whatever else.

These two changes may cause some incompatibilities, too bad, but very nice
in the long term:

Use the power of new the interface in ViewPrivate::setConfigValue
Use the power of new the interface in DocumentPrivate::setViewVariable
With that we will have soon everywhere the same keys and the possibility
literally to set each setting by document variables or the F7 command line
without extra needed work for new options in the future.

Differential Revision: https://phabricator.kde.org/D20924