Summary:
This is particularly useful for cross-compilation, where we only need the
kconfig_compiler on the host system.

Reviewers: #frameworks, apol, aacid

Reviewed By: aacid

Subscribers: aacid

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D6994

Using the new extra-cmake-modules module ECMAddQch (since 5.36.0)
this adds the option to automatically build and install a file
in QCH format with the docs about the public API, which then can be
used e.g. in Qt Assistant, Qt Creator or KDevelop.

Additionally the installed cmake config files will be extended
with a target KF5Config_QCH containing information about how to "link"
into the generated QCH file, which then can be used in the cmake build
system of other libraries building on this library, by
simply listing this target in "LINK_QCHS" of their ecm_add_qch() usage.
And a respective doxygen tag file with all the metadata about the
generated QCH file and used for the "linking" will be created and
installed.

Pass -DBUILD_QCH=ON to cmake to enable this.

Generate-source-file macros should be used from the same CMakeLists.txt
file as the targets they generate files for.

like all the other Qt-based modules do

Drop KCoreAddons and KI18n dependency.
Add a bunch of QStringLiteral()