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
Adds a new open fire sound that is looping for use as the
environment sound effect in the maps who is having fires.

Updated averguard comples map with new events to use the
new env sound effect for fires.
Upaded averguard prison to use the new fire sound.
Updated averguard temple to use the new fire sound effect.
Updated brothers lair atrium to use the new fire sound effect.
Updated brothers lair fire hall to use the new fire sound effect.
Updated goblin warrens to use the new fire sound effect.
Remove section "Running flare" from README.engine
Added enviroment river loop sound effect.

Updated frontier outpost map and placed river sound events
were the river is narrow and under bridges.
Added river sound effects events to frontier plains.
Added river sound effect events to ydrakka pass.
Added river sound effect events to river encampment.
Updated warp zone to use the new fire sound event.
Added new sound effect, forest owl loop.

Updated frontier outpost with a forrest owl placed in tree near
the stash.
Placed a few forest owl soundevents in frontier plains.
Placed a few forest owl soundevents in ydrakka pass.
Added a subtle wind sound effect with some crickets

Updated frontier plains with a global sound event 0,0
with the new wind sound which will always be heard in
constant amplitude.
Updated of frontier outpost map to include the owl sound at stash
added in commit a262153