Fix Formatter library CMake

Summary:
On older versions, install(TARGETS target PUBLIC_HEADER) does not seem
to work too well, so just use install(FILES) for now. In addition, the
Qml and Quick components were missing from the main CMake file.

Test Plan: Still builds and installs

Reviewers: Plasma, romangg

Reviewed By: Plasma, romangg

Subscribers: romangg, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28217