Show Buildhelp should be a display flag

Buildhelp is currently implemented as a boolean. It should be a display flag instead, just like dfShowCensus etc.

Same for draw_resources_, draw_immovables_, draw_bobs_ in the editor.

This will give us a consistent implementation and then make it easier to define preferences, e.g. implementing the buildhelp state into singleplayer saveloading could be done without incrementing the packet version.