Remove po/uh-tutorial/

This completes the move to po/scenarios/ for the tutorial.
Special-case right clicks on trade route slots in res selection mode

Aware of the usability problem that now, under certain circumstances
right clicks will abort the selection process (namely when hovering any
slot, not just the current one) while it won't in other cases. Since I
would like to avoid introducing new mousetools for this rather simple
operation, I hope the pros of this decision outweigh the cons.

Fixes #1310 - Right clicks in resource selection mode should not affect
other slots
Remove pointless messagewidget argument `point=None`

Sorry for the pun. This is the default argument in all relevant
signatures, no need to make code less readable for it.
Also removes a few keywords from mandatory arguments on the way,
namely `string_id` and `messagetext`.