Introduces new class for handling command-line options

The new class drops -h & -v.
The user is expected to use --help & --version.
More modular mjwm.cpp

Moves mjwm procedural code into amm::main class
All classes that need as icon_service can register it after creation
jwm::subcategories always late registers icon_service
Adds TODOs
Fixes regression - ignore desktop files from XDG paths if input directory is specified