Rename player_invention_XXX() to research_invention_XXX(). Move these
functions from common/tech.[ch] to common/research.[ch]. These functions now
take as first argument a 'research' pointer rather than a 'player' one.

See gna patch #4826

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25334 a0f10bec-cc02-0410-94fc-a9cfff90b4cd