OpenGL, zoom, pan and UI restyling

New features:

• GCode preview now is rendered via OpenGL, this allow to zoom and pan the preview
• GCode preview now show engraving progress with different colors
• UI restyling with modern icons, new color scheme, MDI Icons library

This release is pre-release, please report any issue with it.
In menu "Grbl - Settings - Options" it is possible to disable the new features (Legacy Preview and Legacy Icons)