Clean up main CMake scripts.

• Use target based commands instead of source variables and directory

based commands.

• Move find_package calls from top level CMake script into kate and

kwrite subdirectories. This is consistent with the addons where each
target finds its own required dependencies.

• Syntax cleanup: Two spaces indentation and, spaces after command

names and split long commands over multiple lines.

• Append to CMAKE_MODULE_PATH instead of overriding it so that it can

be overriden manually from the command line by developers if needed.

• Remove obvious comments.
• Remove nested project calls as they are unnecessary.
• Rename kdeinit_kate to kate and kate to kate-bin.
• Remove qt5_add_resources usage and pass resource files directly to

target_sources instead (thanks to CMAKE_AUTORC).

• Remove unnecessary and transitive dependencies.
• Exclusively use variables for MacOS bundle strings.