Enhancements for SDL

• Fix: SDL2 using mouse
• Fix: SDL2 controls more reponsive
• Change: SDL2 renderer change for better performance
• Change: SDL examples now use GSLC_DEV_VID_DRV from GUIslice_config.h to set SDL_VIDEODRIVER
• Fix: XSlider control tick drawing
• Change: Makefile added GSLC_LDLIB_EXTRA variable
• Update: GUIslice config guide spreadsheet updates, including Beaglebone Black
• NOTE: Beaglebone operation with SDL is not functional in this release. Please use v0.8.7 instead.
• As always, I welcome suggestions, feedback or code contributions! (guislice @ impulseadventure.com)