theme: Split shared style into shared and fallback

The fact the shared style is loaded at override priority is causing
problems for elementary: #273

While window corners are fixed by the previous commit, they aren't the
only area that they may want to style. Split the part that's just visuals
into a separate stylesheet named fallback.css, and load it at a fallback
priority for all stylesheets, leave shared for critical styles, such as
layout.
13 jobs for master in 17 minutes and 27 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #267117
librem5
api-visibility

00:02:37

passed #267119
librem5
build-debian-clang

00:04:33

passed #267118
librem5
build-debian-gcc

00:03:00

passed #267120
librem5
build-fedora-gcc

00:04:11

 
  Test+Docs
passed #267121
librem5
build-gtkdoc

00:03:16

passed #267122
librem5
unit-test

00:03:24

41.1%
 
  Package
passed #267123
librem5
package-debian-buster

00:02:47

passed #267124
librem5:arm64
package-debian-buster:arm64

00:05:58

failed #267125
flatpak allowed to fail
package-flatpak

00:00:14

failed #267126
flatpak allowed to fail
package-flatpak-glade

00:00:14

 
  Test Package
passed #267127
librem5
autopkgtest-debian-buster-package

00:02:54

passed #267128
librem5
lintian-debian-buster-package

00:01:04

 
  Deploy
passed #267129
librem5
publish-docs

00:00:24

 
Name Stage Failure
failed
package-flatpak-glade Package
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-deck.c.o': Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-header-group.c.gcno': Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-swipe-tracker.c.o': Permission denied
Uploading artifacts for failed job
Uploading artifacts...
_build/meson-logs/meson-log.txt: found 1 matching files

WARNING: sm.puri.Handy.Glade-dev.flatpak: no matching files
Uploading artifacts to coordinator... ok
id=267126 responseStatus=201 Created token=KF35qNSR
ERROR: Job failed: exit code 1
failed
package-flatpak Package
rm: cannot remove '_build/meson-logs/coveragereport/builds/Librem5/libhandy/tests/test-window-handle.c.func.html': Permission denied
rm: cannot remove '_build/meson-logs/coveragereport/builds/Librem5/libhandy/tests/test-combo-row.c.func.html': Permission denied
Running after_script
Uploading artifacts for failed job
Uploading artifacts...
_build/meson-logs/meson-log.txt: found 1 matching files

WARNING: sm.puri.Handy.Demo-dev.flatpak: no matching files
Uploading artifacts to coordinator... ok
id=267125 responseStatus=201 Created token=1fzKs7UD
ERROR: Job failed: exit code 1