Added more audio, font and FS methods. Some improvements and fixes.

- Added sound/music volume controlling methods.
- Added sound playback speed/pitch controlling methods (including a
cheap SDL implementation as well)
- Added filesystem functions to check if path is file/directory and
gather current working directory.
- Renamed Font::getSize to Font::getFontHeight(). Added method to
compute text width with a given font.
- Added Menu class to the extras.
- Partially fixed music pausing on SDL adapters.
- Fixed leaks in fgeal::getFilenamesWithinDirectory() on Allegro
adapter. Added failure check.
- Fixes to compile on MinGW
- Moved filesystem utility functions to core_agnostic.cpp. Now
implementation for these are now adapter-agnostic and is based on
stat.h, dirent.h and direct.h/unistd.h. Allegro 5.0 adapter's
implementation using Phyfs addon was commented out. This should remove
Allegro 5.0 adapter dependency on the Phyfs addon (and all involved
building around it). Now dirent.h is a dependency for all adapters.
- Overall improvement of SDL adapter's font and audio code. Cached SDL
audio specs to improve speed.
- On SDL 1.2 adapter, added macros to enable IMG_Init/Mix_Init only with
supported versions.
- Created experimental MSVC build configurations for eclipse.
- Overall improvements to eclipse build configurations.
- Other minor fixes.