[libadwaita] (5 commits) Non-fast-forward update to branch wip/exalm/backdrop

[Alexander Mikhaylenko <alexm src gnome org>]

The branch 'wip/exalm/backdrop' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  8797052... stylesheet: Inline headerbar_fill()
  b7595e3... stylesheet: Use a flat gradient for backdrop header bar
  cff05b7... stylesheet: Remove redundant header bar rules

Commits added to the branch:

  3ba1ac3... stylesheet: Inline headerbar_fill()
  6f81be5... stylesheet: Adjust devel header bar gradient
  50a3685... stylesheet: Use a flat gradient for backdrop header bar
  35476ce... stylesheet: Remove redundant header bar rules
  dbedb2b... stylesheet: Make the .devel cog less prominent for backdrop