Unload sounds when destroying MapRenderer

This fixes environment sounds continuing to play after exiting to the
title screen from gameplay.
Increase preallocate mixing channels.
Make use of location based sound for enemy and loot.
Merge pull request #391 from dorkster/bugfixes

Unload sounds when destroying MapRenderer
Merge pull request #393 from stefanbeller/master

cleanup Readme, enhance install instructions.
Merge pull request #394 from hean01/master

Increase of preallocated mixing channels.
Pause all env. sound that is out of distance to go easy on
SDL mixer, this will decrease mixing of 30 events to 2-4.
Implement enemy-style loot tables for map events

The new formatting for the loot property in map events is as follows:

```
item id # or 'currency', x, y, quantity, percentage chance
```

An item id of 0 is equal to 'currency'.

This change has the added benifit of allowing items to drop in stacks of
more than one (e.g. 5 Health Potions).
Lock the Action Bar when transforming

Closes #401
Allow buff powers to use post_power

An example usage of this is to have a passive "buff" that has an ON_HIT
trigger to untransform the hero.
Don't allow equipment changes when transformed
Made the local data prevail upon installed ones.

This should be the case when no custom game data folder has been set,
and should permit to run the engine without the need to install it,
at least on linux.

fixes flare-game issue 160.
Merge pull request #404 from Bertram25/master

Fix for flare-game 160
Merge pull request #398 from hean01/env_sound

Pause all env. sound that is out of distance to go easy on
Merge pull request #399 from dorkster/map_loot

Implement enemy-style loot tables for map events
Merge pull request #402 from dorkster/transform

Lock the Action Bar when transforming
Made env. sound falloff configurable thru engine/misc.txt using
key sound_falloff, the default settings value is 15 but can now
changed.
Fixes deletion of saved games to use game_prefix if used.
This closes issue #406
Merge pull request #405 from hean01/master

A few fixes.
GameStateTitle: ease of logic.