Implementation of MenuBook

Original author: Igor Paliychuk
Don't close other menus when opening books
Prepare for sfx refactor
Add level up sound path to StatBlock
Deprecate the use of sfx_prefix and make Avatar sounds configurable

Avatar sound configuration is done in `engine/avatar/$GFX_BASE.txt`
Allow modding footstep sound effects

Footsteps are configured in `items/step_sounds.txt`
Merge pull request #1005 from dorkster/sounds

Remove sound effect hard-coding
Merge remote-tracking branch 'upstream/master' into book_patch
Remove triggering books via events
Fix memory leak in CombatText
Merge pull request #1002 from dorkster/book_patch

Implementation of MenuBook
Merge pull request #1006 from dorkster/fixes

Fix memory leak in CombatText