panel: Read gsettings for solid or transparent style
Because this MR removes panel transparency but many people love it, I modified some code to make panel switchable between transparent or solid using a gsettings key
org.gnome.desktop.interface.keep-panel-solid. By default it is solid as GNOME 3.32, and can be changed to false for transparency.
Will add an option in GNOME Tweaks so user who cannot see text clearly in transparent mode with a light wallpaper can set panel to solid, while others set panel to transparent.
Depends on gsettings-desktop-schemas!18 (closed).