Mostly general cleanup, also the following changes:
- Disabled flat buttons will have a fully transparent background (before
it was a partially transparent white background)
- Disabled raised buttons have a (light) grey background and (darker)
grey text. There is no difference in the disabled styling whether the
button is primary or not.
- Add loading as a property on gr-button, translate that into disabled
on the paper-button, which removes some CSS rules.
Bug: Issue 7496
Change-Id: Iae0564d099508827c8f13fbb943c6646d631ce1d