widgets: checkbox.rs update
widgets: checkbox.rs update

* reflact usage of new ttf 'material_icons_font_ttf'
* use new Glyph syntax: 'MD_CHECK'

Signed-off-by: Ralf Zerres <a55c5beace021459791f5070a9a90148cd697b8f@networkx.de>

redox-os/orbtk