Introduce a setting 'audio' in the config file to disable audio.

Currently this setting cannot yet be set in the userinterface config.
Having this option available, allows us to not call Mix_OpenAudio
if audio is completely disabled, so the audio subsystem is not initialized
at all.
Merge remote-tracking branch 'mainline/master'
Capsule the audio processing better into their own functions.
Shrink scope of variables.
remove unneeded assignments.
PowerManager: complete init list.
Add Gallaecio to mailmap file.
README.engine: move installation related text into seperate INSTALL.engine file.
Update of swedish translation.
Merge pull request #313 from stefanbeller/master

Introduce a setting 'audio' in the config file to disable audio. 
Merge pull request #315 from dorkster/bugfixes

Fix MessageEngine always being initialized as English
Merge pull request #317 from hean01/translation

Update of swedish translation.
Merge pull request #316 from matthiaskrgr/master

    README: move installation related text into seperate INSTALL file.
Fix issue #318 combat text wrong location
Avoid div by 0 on random currency loot
Fixes compiler warnings about shadowing global declaration
of screen.
Merge pull request #320 from hean01/bugfixes

Fixes compiler warnings about shadowing global declaration of screen.
Removed duplicated code logic and free allocated WidgetLabel object.
Fixes #319.
Merge remote-tracking branch 'upstream/master' into bugfixes

Conflicts:
	src/CombatText.cpp
Allow the keyword "currency" in enemy loot entries