SDDM: update for the new AeroShell SDDM KCM
Update installation script to install the new AeroShell SDDM KCM
Look and feel + Shell: switch to AeroShellUtils for the SDDM background
This commit overhauls the repository structure such that it separates
the project into multiple repositories, one for each subproject. This
repository will be moved to the AeroShell group, and most importantly,
the installation method changes from locally installed modifications, to
CMake-based installation, in preparation for AUR packages, and possibly
packages for other distros.
Migration details are in INSTALL.md, which are highly recommended, if
not required to uninstall any old instance of AeroThemePlasma.
