Add a couple of find modules for wayland-scanner and qtwaylandscanner.
These modules find the respective executables and create a target that
points to the executable.

Targets are respectively Wayland::Scanner and Wayland::QtScanner.

There are also macros to generate C protocols with wayland-scanner and
C++ wrappers with qtwaylandscanner.

REVIEW: 120034

add FindLibGit2.cmake + doc link
scripts tries to detect version not by pkgconfig to work on windows without pkgconfig, too
REVIEW: 120196

ecm_create_qm_from_po_files() was actually not very useful in practice.
So that is deprecated, to be removed before ECM 1.0.

Instead, the ECMPoQmTools provides several useful functions:
ecm_create_qm_loader() (which already existed in
ECMCreateQmFromPoFiles), ecm_process_po_files_as_qm() (which has the
same signature as gettext_process_po_files() from the FindGettext
module) and ecm_install_po_files_as_qm(), which is a convenience
function mostly for the benefit of KDE Frameworks (although potentially
useful for whatever other projects have the unusual requirement of a
Gettext translation workflow but no Gettext usage in the code).

NB: some clean-up to the documentation was done by Alex Merry
<alex.merry@kde.org> as part of this commit.

REVIEW: 117823

The ecm sphinx module occasionally tried to modify a list it was
iterating over, which is a Bad Thing and raised an exception on the
third or so time it was run without clearing the build directory.

REVIEW: 117780

ecm_dbus_add_activation_service() requires suffient knowledge of its
internals to use that replacing two lines with one seems silly.

In order to use it you have to know it behaves like configure_file()
(because you have to construct the file yourself), except that it also
installs it somewhere (for which you have to make sure
DBUS_SERVICES_INSTALL_DIR is defined before you use it, which is
certainly not a given for non-KDE projects). By this point, why not just
use configure_file() and install()? The DBUS_SERVICES_INSTALL_DIR
provided by KDEInstallDirs is all the magic you actually need, and if
that's explicit in the CMakeLists.txt file, it's a lot more obvious that
you should have it defined somewhere.

REVIEW: 117581

This adds an ecm-developer manual that replaces writing-find-modules.md
(a lot of which was upstreamed to CMake's own documentation). It also
adds introductory text to the ecm-*-modules manuals.

REVIEW: 117624

This is deliberately modelled very closely on CMake's documentation
system.  It's a hefty patch, because it involved changing all the
documentation to be in reStructuredText format.  I also cleaned up the
copyright/license statements at the same time.

Note that the find modules contain the full license, due to the fact
that ecm_use_find_module() copies them out of the ECM distribution.