- point to main Calamares site in the 'part of' headers instead
  of to github (this is the "this file is part of Calamares"
  opening line for most files).
- remove boilerplate from all source files, CMake modules and completions,
  this is the 3-paragraph summary of the GPL-3.0-or-later, which has
  a meaning entirely covered by the SPDX tag.

The intended license for the CMake modules is BSD-2-Clause;
there's no desire to restrict what can be done with these,
and CMake modules are generally 2-clause licensed.

- Nothing ever sets LIBRARY_QT5_MODULES (it would be a macro
  argument to calamares_add_library, if anything).

- Add the Calamares CMake-modules to the search path automatically
- Export to CalamaresTargets.cmake and use namespace Calamares::
- Document imported targets
- Find Qt, because the translations machinery will need macros from that
- The installed lib links to IMPORTED libraries from KF5, so we need
  to find them (again) as well.

Update CalamaresAddTest.cmake

Update CalamaresAddLibrary.cmake

Update CalamaresAddBrandingSubdirectory.cmake

Update CalamaresAddTranslations.cmake

Update CalamaresAddPlugin.cmake

Update CalamaresAddModuleSubdirectory.cmake

 - Use autouic so that we can also pass in --include to add
   a code-warning-suppression to the generated code, just like
   we can do with moc.

 - The auto-generated code produces a lot of warnings from
   Clang 8; this obscures the more meaningful warnings from
   actual Calamares code, so tone the warnings down.
 - For Clang, set CALAMARES_MOC_OPTIONS.
 - Add convenience CMake function for automoccing. It applies
   the options as needed to a given target.

 - Remove the commented-out cruft and the whinging
 - Fix use of COMPILE_DEFINITIONS with a list passed in
 - Remove unused arguments (TYPE, TARGET)
 - Document calamares_add_library
 - Document how to use COMPILE_DEFINITIONS (in calamares_add_plugin)

Since we install the CMake files now, also put license headers in them.
These files are intended to help plugin and branding component authors
write them in a simple Calamares-idiomatic way.

This also adds ImageRegistry, a SVG cache, a bunch of utility functions
for shared pixmaps in CalamaresUtilsGui and renames several qrc files
in viewmodules.

Now linking with -Wl,--no-undefined!
Moved find_package KF5 from the root CMakeLists.txt to the partitioning
module where it belongs. KF5 deps should be limited to plugins.
Lots of random fixage to make stuff link.
Removed CalamaresApplication reference from Settings.
Removed the use of AbstractPage, we don't need it right now.

The qt_* macros have a different signature

Added ViewManager.
Added dummy Settings class.
Added dummy plugin interface (UI plugins only).
Added dummy greeting plugin.
Added DLLEXPORT macros for UI plugin interface and plugins.