Add GLUI slider. Closes #3

- Added GLSlider class which creates an OpenGL Slider. Given an upper bound and lower bound the slider will calculate the current value represented by the slider's position relative to the slider track's position.

- Added the option for foreground colours to GLUITheme.

- Updated TestGLUI to show new features.
Merge branch 'master' of github.com:ayrtonmassey/OOP-Project