Tweak in the new workspace creation dialog to copy settings from an existing one

[frivoal]

In the recently introduced UI to create or switch between workspace (see #19516), I'd find it convenient if I could clone the config of an existing workspace when creating a new one, instead of starting from scratch. I'd still want the library/database to be separate, but starting with the same settings as what I've already tweaked to my liking in another workspace would be a huge time saver.

This could be achieve with a "duplicate" button next to the existing ones, or with the "clone config from…" dropdown below the name field for creating a new workspace, where you could pick one of the existing workspaces (likely with default preselected), or none.

[wpferguson]

The darktablerc file is now split in 2, one for common settings and one for workspace specific settings. So any settings that are common, should be saved in the darktablerc-common and available across all workspaces.

[TurboGit]

@wpferguson : You're right, but this is an implementation details and the darktable-common only contains a very limited set of settings needed for starting dt and having the workspace dialog with the proper UI. So not a solution here.

[frivoal]

I'm not sure where the split is exactly between the two parts, but there are a number of things that I would expect to be shared or easily clonable which aren't. For example: lighttable/expand a single module at a time, or `miscellaneous/mouse wheel scrolls modules side panel by default".

It seems to me that any UI-related setting are more likely to be prefered to be the same across workspaces than preferred to be different. I don't care very strongly if that's achieved through direct sharing, or through copying at workspace creation, but I think one of the two would be useful.