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 .
Action icons: add gtk-ok.svg as a symbolic link of dialog-ok.svg

The icon-naming-utils utility doesn't take care of this alias, so we
do the symbolic link in the Makefile.  The alias is needed for stock
icons with Gtk.STOCK_OK id.

Fixes #4010 .
Add Apache license