QmlPlugins macros force installation in LIBDIR

The original QmlPlugins macros required one to set DESTINATION for a plugin to be installed at all.

Today they force it to be $LIBDIR/qt5/qml:

http://bazaar.launchpad.net/~cmake-extras/cmake-extras/trunk/view/head:/src/QmlPlugins.cmake#L110

This prevents usage of QmlPlugins for private or mock plugins that need to be installed into a private import path, if at all.

DESTINATION needs to be respected, and if it needs to default to ↑, there needs to be a new option to disable installation.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: cmake-extras 0.5+16.04.20160113-0ubuntu1
ProcVersionSignature: Ubuntu 4.3.0-7.18-generic 4.3.3
Uname: Linux 4.3.0-7-generic x86_64
ApportVersion: 2.19.4-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Jan 29 01:06:42 2016
Dependencies:

PackageArchitecture: all
SourcePackage: cmake-extras
UpgradeStatus: Upgraded to xenial on 2015-10-28 (92 days ago)