Play item sounds in MenuVendor

This plays sounds when items are returned/added to either of the vendor
tabs.
Remove cppcheck-warnings from sound manager and harmonize coding style.

[src/SoundManager.cpp:59]: (style) The scope of the variable 'psnd' can be reduced.
[src/SoundManager.cpp:42]: (warning) Member variable 'Sound::chunk' is not initialized in the constructor.
[src/SoundManager.cpp:152]: (performance) Prefer prefix ++/-- operators for non-primitive types.
Merge pull request #361 from stefanbeller/master

Remove cppcheck-warnings from sound manager and harmonize coding style.
Merge pull request #358 from dorkster/bugfixes

Play item sounds in MenuVendor
Remove hardcoded sound fx references from MenuManager and
open/close sound effects are defined in menu/menus.txt.

Part of #357
Remove menu close sound effect responsibility from closeLeft/Rigth/All()
and play the correct menu close sound effect at spot.
Add playback of close sound effect for powers menu.
Move hardcoded sound reference of coins and loot sound effects
into engine/loot.txt.

Added a LootManager::playCurrencySound() to be used by core for
currency sound which replaces the old ItemManager::playCoinSound()

Part of isse #357
Added game_prefix in engine/misc.txt for ability to segregate saved games
and stash for different games/mods.

Removed duplicated code for stash loading.
Merge pull request #363 from hean01/master

Remove of hardcoded sound effect references in MenuManager
Merge pull request #366 from hean01/game_prefix

Added game_prefix in engine/misc.txt for ability to segregate saved games