modulegroups: right-click should adjust current preset if it is adjustable

Currently, if a user right-clicks on a module group icon in the darkroom and adds or removes a module or widget, this automatically creates a new preset named "last modified layout", to which the adjustment is made.

While this is fine (and is the only reasonable behaviour) when the user has selected a read-only preset, it creates an annoying workflow if they are working with a user-defined preset - the user has to go into the preset menu and either adjust their existing preset to match or delete their existing preset and rename "last modified layout".

Please can this be changed so that the "last modified layout" preset is only created or altered when adjusting read-only presets.