fix a && that should have been a ||
prettify manager settings dialog
hack: no button reaction when clicking manager button

It sometimes stayed deactivated after closing the dialog.
get rid of "industry prio" option

This means that the industry slider is now always active. The old
industry worker distribution did basically not differ from the new one
when setting the industry slider to CSystemManager::max_priority
(since the introduction of the feature to try to use all buildings
in case of max_priority) so we can simplify code and UI.
put food workers again on top in DoRemaining()

Not sure about whether there should be a slider for food,
so people can indirectly put there workers that way.
increase max_priority again to 40

Roughly the border between middle-sized and large systems.
This should generally suffice to prevent "wrong" worker distribution,
if priorities are set in a sensible manner (at least one other category
is on max), so a secondary fallback category would get 1/39 of the
workers of a primary one.