Added stub ruleset under version control

See patch #4362


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25199 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Instead of returning NULL surface, sdl2-client's create_icon_theme_surf()
creates somewhat dummy surface

See patch #4810


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25200 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added daieffects.[ch] module for AI evaluation of effect values. It contains
old improvement_effect_value() refactored and now called dai_effect_value() along
with some of its helper functions.

See patch #4813


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25201 a0f10bec-cc02-0410-94fc-a9cfff90b4cd