Further cleanup addons CMake scripts.

• Update CMake minimum required version to CMake 3.1 which is when

target_sources was added.

• 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.