To have the code bits reusable, they are defined as cmake variables.
In order to make the relative paths work and avoid touching every line
from the QRC, the resulting file is put into the source dir.
On Plasma qqc2-desktop-style is used automatically, but not on other
DEs. Set it as a default and allow overriding it via environment
variables.
Also add it as a runtime dep so distros actually ship it.
Summary:
* cmake 2.8.12 is really outdated, 3.0 is minimum in plasma & kf5
* cmake_minimum_required should be at begin of toplevel CMakeLists.txt
* bump qt version to 5.7, matching the min Qt version of kf5 5.42
* with ecm being part of kf5 since early versions, share ${KF5_MIN_VERSION}
* use QT_MIN_VERSION & KF5_MIN_VERSION also for separate find_package calls
* deduplicate all KF5 components searched for in unconditionally included
subdirs
Test Plan: Still configures and builds with all options OFF & ON
Reviewers: #kde_connect, nicolasfella
Reviewed By: #kde_connect, nicolasfella
Subscribers: nicolasfella
Differential Revision: https://phabricator.kde.org/D11418
