GTK3 theme 72%, fix chekboxes

- We had 'indicator-size' harcoded to 26px.  Now is a proper number
  for sugar-72 theme.

- Provide new SVG files for the checkbox states, for sugar-72 .  Those
  were optimized with SVGO https://github.com/svg/svgo

- Reorder the assets in directories 'assets/100' and 'assets/72'.  The
  Makefile takes care of copying the correct SVG files to the build
  directory.

Fixes #4092 .
GTK3 theme 72%, fix sliders

- Variable scale_trough_margin was harcoded for both themes.  Now is a
  proper number for sugar-72 theme.

- Provide new SVG files for the slider assets, for sugar-72 .
GTK3 theme 72%, fix radio buttons

- Provide new SVG files for the radiobutton states, for sugar-72 .