In the ubuntu 20.04, we found a regression on HDMI audio. We plug a
HDMI/DP monitor, the output device is the speaker in the output device
combo box, then we select the HDMI from the combo box, the output
device is switched to HDMI audio successfully, but the configuration
box is empty. This will not happen in the ubuntu 18.04, the profile
will show hdmi-stereo in this situation. So I think this is a
regression.

When problem happens, I could see the errors from log:
Gvc:    DEBUG: Matching profile for 'output:analog-stereo' is '(null)'
Gvc:    DEBUG: Matching profile for 'output:analog-stereo' is '(null)'
Gvc:    DEBUG: Matching profile for 'output:analog-stereo' is '(null)'

Through debugging, I found the cc_profile_combo_box_set_device() is
called too early. In the ubuntu 18.04, this function is called after
pulseaudio changes the active_profile, but in the ubuntu 20.04, it is
called ahead of pulseaudio changes the active_profile.

To fix it, add a signal callback function, after pulseaudio changes
the active_profile, this callback function will be called, then call
cc_profile_combo_box_set_device().

Signed-off-by: Hui Wang <hui.wang@canonical.com>

In the shortcut editor, the "Replace" button is shown instead of "Set"
or "Add" when there is a collision. But this executes the same code as
"Set", so when adding a shortcut, it doesn't actually work.

The dialog is not a GObject, so this didn't work. It was converted to a GObject
in master in 26e8f115.

The reverts part of 4502673b