Draft: config, dialog, widgets: preference option to show mode groups
Adds a preference option:
Preferences->Interface->General->Show available mode groups button
When unchecked, the buttons next to the layer modes box and paintbrush mode box are hidden. This makes for a simpler GUI, especially if the user never uses that feature. For the layer modes, the option is still available to select in the layer attributes dialog.
I had to tackle this two ways, the preferred way was to pass an option to 'gimp_layer_mode_box_new'. This didn't go down so well with the layer tree, as I could only access the gui_config after the layer tree had been created. So for that box I had to add a function to access the created mode group box, so that it could be hidden post creation.