org.mate.panel.toplevel.background: glib-compile-schemas doesn't compile panel "color" unless "type" is set

# gsettings (control)
* Check the default system settings in Ubuntu MATE 23.04:
```
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color
# Output:
# '#ffffff'
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ type
# Output:
# 'none'
```
* Set a panel background with gsettings, then verify it:
```
gsettings set org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color '#00ff00'
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color
# Output:
# '#00ff00'
```
* Reset the color: `gsettings reset org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color`

# glib-compile-schemas doesn't compile it
* Run this in a terminal:
```
sudo tee /usr/share/glib-2.0/schemas/zzz.gschema.override > /dev/null << 'EOF'
# green mate-panel background color. type='color' must be set in addition
[org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/]
color='#00ff00'
EOF

sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
```
* No error was reported by glib-compile-schemas
* Log out, and log in from MATE
* Run this
```
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color
# Output:
# '#ffffff'
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ type
# Output:
# 'none'

####################
# Reset to the new system wide default settings:
gsettings reset-recursively org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/

# Check the values again to compare them with their previous output:
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color
# Output:
# '#ffffff'
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ type
# Output:
# 'none'
```
* As you can see, the values don't change. My under

## Workaround: Include type='color'
* Run this in a terminal:
```
sudo tee /usr/share/glib-2.0/schemas/zzz.gschema.override > /dev/null << 'EOF'

# green mate-panel background color. type='color' must be set in addition
[org.mate.panel.toplevel.background]
color='#00ff00'
type='color'

sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
```
* No error was reported by glib-compile-schemas
* Log out, and log in from MATE
* Run this
```
# Check the values:
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color
# Output:
# '#00ff00'
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ type
# Output:
# 'color'

####################
# Reset to the new system wide default settings:
gsettings reset-recursively org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/

# Check the values again to compare them with their previous output:
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ color
# '#00ff00'
gsettings get org.mate.panel.toplevel.background:/org/mate/panel/toplevels/top/background/ type
# Output:
# 'color'
```
* As you can see, the values are changed.

## Why the color variable should not be required
The color is selected even if the style is changed to "none"


# Distributions
Ubuntu MATE 22.04:
```
$ apt-cache show libglib2.0-bin | head -8
Package: libglib2.0-bin
Architecture: amd64
Version: 2.72.4-0ubuntu2.2
Multi-Arch: foreign
Priority: optional
Section: misc
Source: glib2.0
Origin: Ubuntu
```

Ubuntu MATE 23.04:
```
$ apt-cache show libglib2.0-bin | head -8
Package: libglib2.0-bin
Architecture: amd64
Version: 2.76.1-1
Multi-Arch: foreign
Priority: optional
Section: misc
Source: glib2.0
Origin: Ubuntu
```

# Upstream issue link
https://github.com/mate-desktop/mate-desktop/issues/562