Summary:
Having to manually maintain a separate copy of all the data about qt logging
categories in the categories files comes with the usual disadvantages.
The new macro ecm_qt_install_logging_categories together with the additions
of arguments DESCRIPTION & EXPORT to ecm_qt_declare_logging_category allows
to have just one place with one copy of the data, and have the categories
file automatically generated from that data, linked via the EXPORT id.

For cases not using ecm_qt_declare_logging_category, but having categories
manually defined in code, yet wanting to have info about those categories in
the installed fiel, ecm_qt_export_logging_category allows to add those data
to the system.

Test Plan:
Added unit tests work, porting of some repos created categories files whose
diff against the manually created files were only the DO_NOT_EDIT header.

Reviewers: #build_system, #frameworks, broulik, mlaurent

Reviewed By: mlaurent

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D27150

This makes life a bit easier for developers who use the categorised
logging in Qt5 in the common case - rather than creating two new files,
and remembering to put in the #ifdef for the default verbosity settings
in Qt 5.4, they can just add a couple of lines to their CMakeLists.txt.

REVIEW: 124595