Refactored a lot of things and done minor fixes.

- Add MSVC project files.
- New revamped Vector2D struct and new geometry.hpp header.
- New filesystem.hpp with all filesystem helper functions.
- New timing.hpp header with timing related functions.
- Version macros are now in version.h
- Renamed Display::existInstance to Display::wasCreated
- Display::create now returns a reference.
- Changed some Display parameters and return types to unsigned.
- Disabled singleton classes' copy constructor and value-assignment
operator.
- Moved checkInit to core class. Added optional macro to make it empty.
- Display::getInstance no longer creates and instance. Use
Display::create beforehand.
- Moved initialization and deinitialization macros to fgeal.hpp.
- Keyboard::Key and Mouse::Button are now simple enums instead of
struct.
- Made Event::Type a simple enum instead of a wrapper struct.
- Fixed Image::drawCircle() from SFML 2.x adapter.
- Partial fix for SFML 2.x adapter font height estimation.
- Fixed SFML 1.x adapter font height estimation.
- Renamed some SFML 1.6 compatibility macros.
- Replaced WINSDK_INCLUDE_PATH & WINSDK_LIB_PATH build variables with a
single one: WindowsSdkDir.