implement system prompter

Closes #19

See merge request Librem5/phosh!67

This gives us system modal dialogs for all important pin entries (if
they use Gcr as they should).

Based on gnome-shell's shell-keyring-prompt.c as well as Gcr's
own ui/gcr-prompt-dialog.c

Closes: #19

This makes e.g. gcr system prompter happy but other things like
services trying to connect to g-s-d need it too.

This should increase readability and make it simpler for folks not
familiar with the script.

settings: Make brightness slider work correctly

See merge request Librem5/phosh!90

Clicking on the slider worked but dragging it resulted in a flickering
display. This works now but beware of

    https://github.com/swaywm/wlroots/issues/613

Start phosh via gnome-session

Closes #23

See merge request Librem5/phosh!83

This is not good for the running session.

Things like gnome-terminal require an UTF-8 locale and although we
expect gnome-session to set this up it's better to have a UTF-8 locale
right away.

This makes sure other packaging formats get the configuration too.

Phosh is of little use without it.

Instead of launching rootston directly spawn it via /usr/bin/phosh and
make it run the session.

Closes: #23

Install it to /usr/share/gnome-session/ so GNOME's gnome-session
can find it.

gnome-session starts applications by desktop file.

We set X-GNOME-AutoRestart=false for the moment to make it simpler to
spawn new development versions. We might change that at a later point
for release builds.

gnome-session needs an X11 display so start rootston first and make it
spawn gnome-session (which will spawn phosh later on).

Preface all LayerSurface namespaces with "phosh"

See merge request Librem5/phosh!89

This makes sure our rootston extension can find it. Name the panel
itself "phosh" since this surface's output is being rotated.

Add home button

Closes #27

See merge request Librem5/phosh!88

Just the button to spawn the favorites. Favorites, top anel rework will
be done separately (See #24, #29)

Closes: #27

The convention is to have the _TYPE_ after the prefix

Turn panel into a PhoshLayerSurface instead of open coding it

Closes #21

See merge request Librem5/phosh!87

Closes: #21

Introduce PhoshLockscreenManager

Closes #25

See merge request Librem5/phosh!85

This makes it configurabe via gnome-settings. But see #28 where this has
to go.

This is one open coded layer surface less and another part
of #21.

Closes: #25

This gives us better backtraces

Fix various potential crashers and make avoiding them simpler

See merge request Librem5/phosh!86

otherwise the unitialized variable might be cleaned up on the error
path.

Be pessimistic and assume failure when the pam handle did not get
cleanued up earlier.