Adwaita: circles for .circular buttons

- doesn't catch all cases, but most buttons need uniform paddings
  to remain circular, not elliptical.

fixes issue #1695
2 jobs for gtk-3-24 in 9 minutes and 18 seconds
Status Job ID Name Coverage
  Build
passed #224352
build

00:09:18

manual #224353
win32 allowed to fail manual
msys2-mingw32