New part/ directory for okularpart sources from /, ui/, conf/

Test plan

• Local build with OKULAR_UI:STRING=both
• GitLab pipeline

Suggested commit message

New part/ directory for okularpart sources from /, ui/, conf/

Source files are no longer separated by UI and non-UI and similar,
but only by their build target.

* ui/ -> part/
* Move all source files from conf/ to part/
* Keep config skeleton definitions in conf/, needed for the mobile target too
* Move editdrawingtooldialogtest.h from conf/autotests/ to autotests/
* ui/data/icons/ -> icons/
* Move /part.cpp, /part.rc and similar files to part/
* Adapt include paths in source files
* Adapt CMakeLists.txt files (in / and in subdirectories)
* Adapt /Messages.sh