daemon: Also store name, color, and avatar in dashboard configuration

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Update shell.hosts to new dashboard configuration storage

This takes color and avatar directly from the dashboard configuration,
and uses the name in the dashboard configuration as a cache for the
pretty name of the target host.

Also allow setting of the pretty name via shell.hosts.

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: New shell.image_editor

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Server appearance edit dialog for dashboard

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Styling for multi-server dashboard list

Reviewed-by: Stef Walter <stefw@redhat.com>
daemon: Remove support for local avatars

Closes #1467
Reviewed-by: Stef Walter <stefw@redhat.com>
bridge: Only do base64 decoding in one place in CockpitChannel

There's no need to do this in two places.
bridge: Add a prepare vfunc to CockpitChannel

The queueing of cockpit_channel_ready() and cockpit_channel_close()
was too magical and causing too much complexity.
bridge: When we receive clean close before ready, don't close yet

When we received data or a clean close before the channel is ready
don't close prematurely. Make sure all the data is drained, and
then do the clean close.
bridge: CockpitChannel "disconnected" when transport closes

The CockpitChannel closes unsuccessfully when the transport
closes. This lets implementations react immediately, even if the
transport closes without error.

Closes #1518
bridge: More robust parsing of channel options

The parsing of channel options was quite haphazard, and didn't
detect bad types for values, etc. This is more verbose, but it's more
failure proof and produces better diagnostic messages.
bridge: Better diagnostics for file read/write failures

In the file read/replace channels, put some context in the messages
so we can have a clue as to what's causing it. Previously we had:

cockpit-bridge-Message: /tmp/.V3DRPX/foo: Bad file descriptor