Add new icons and panels Grease Pencil Dopesheet to manage layers without having the properties panel visible.
Also, the icons are in the same order in Dopesheet, Layers and Material list to keep consistency.
As the number of columns for icons is limited to 3 and we also need use a factor, I have impleted the change using slider area. Also, the slider option is enabled by default for 2D Template.
See T72026 for more info.
Note: I would like to include this in 2.82 version