Require Qt 5.7 and remove pre-5.7 code

Summary:
Increases minimum Qt version to 5.7. This allows to drop the pre-5.7
virtual keyboard and various ifdefs for now unsupported versions.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D4485