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
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.
GameStatePlay::checkLoot quits early when the hero is dead
Calculate derived stats on the fly instead of caching those.

This was cherry-picked from the previous attempt to reduce
the StatBlock class.

Conflicts:
	src/GameStatePlay.cpp
Make SoundManager support playing sound on locations, were distance
from location to pc controls the amplitude, eg. the further away the
sound is the more faint it is played out.

Made Map events on_load special for looping a sound effect at the
position of the event, this will be used for location based env.
effects like the fires we now have in warp zone and atrium.
Merge pull request #387 from stefanbeller/master

Calculate derived stats on the fly instead of caching those.
Rename sfx to soundfx for consistency in data files.

Fixes #388
Merge pull request #389 from hean01/env_sound

Support for location based environmental sound effects.
Merge pull request #390 from hean01/soundfx_attribute

Rename sfx to soundfx for consistency in data files.
Unload sounds when destroying MapRenderer

This fixes environment sounds continuing to play after exiting to the
title screen from gameplay.
Fix linear, topic-less dialogs
Merge pull request #392 from dorkster/npc

Fix linear, topic-less dialogs
Remove section "Running flare" from README.engine
Install instructions: add fedora dependencies.
installation instructions: Remove python and standard gcc for openSUSE

There is still gcc-c++ which is enough.
Increase preallocate mixing channels.
Reindent the SoundManager with tabs, add comment to AUDIO variable.