Adwaita: allow checked tab to look connected to the netwbook

In current implementation, the checked tab look a bit disconnected from
the relavive notebook. This small rework let the active tab looks like
it is continuation of the relative tab.
5 jobs for rework-gtk-tabs in 11 minutes and 12 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #344928
fedora-x86_64

00:07:32

passed #344929
win32
msys2-mingw32

00:11:12

 
  Flatpak
manual #344930
allowed to fail manual
flatpak-manual:demo
manual #344932
allowed to fail manual
flatpak-manual:icon-browser
manual #344931
allowed to fail manual
flatpak-manual:widget-factory