Ignore power id 0 when saving/loading power list

A power id of 0 means no power, so there is no point in adding that id
to the powers list. Also, the size of the powers_list vector itself
should be 0. Previously, it had a initial size of POWERSLOT_COUNT (10).
This meant that the `powers=` key in save files would infinitely expand
because we kept adding 10 zeros to that list every time the player saved.
Fixes build warnings about coupled && and || without parentheses
Merge pull request #385 from hean01/master

Fixes build warnings about coupled && and || without parentheses
Make "Shop" and "Cancel" action labels configurable / tranlatable.
Merge pull request #384 from dorkster/bugfixes

Ignore power id 0 when saving/loading power list
Merge pull request #386 from hean01/npc_actions

Make "Shop" and "Cancel" action labels configurable / translatable.