utf-8-bom for better compatibility for some programs

have launch.py examine both COMMANDLINE_ARGS as well as argv for its arguments

Don't eat colons in booru tags

Added dark mode switch

Added possibility to set the theme (light or dark)

Launch the UI in dark mode with the --dark-mode switch

also useful for keeping settings cache and restore logic together, and nice for code reuse (other third party scripts can import this class)