Further cleanup addons CMake scripts.

• Use target_sources even when commands that work on sources variables are used. Instead of appending the generated sources to an existing sources variable, we store them in a temporary variable and immediately add them to the corresponding target with target_sources.
• Remove unnecessary dependencies on kdeinit_kate.
• Remove the intermediary katefiletree static library and have the tests build the single required source file manually instead which is consistent with how the other addons with tests work.