GUIDialogSelect - add optional 3rd button

[DaveTBlake]

Add an optional 3rd button to the standard select dialog GUIDialogSelect
Update Estuary and Estouchy skins to be able to show this button (id=8)

An example use is in UI for entering multi-value settings where the user can either pick from a default list or add a new value, but other uses may arise in future.

See #18173 where it will be used for the artist and album art whitelist settings that offer multiple selection of art types, enabling the user to add own new art types. Similar applies to video art.

Other skins will need to be modified similarly, not sure where that needs to be documented or how best to notify skinners. @ronie perhaps you can advise.

[ronie]

Other skins will need to be modified similarly, not sure where that needs to be documented or how best to notify skinners

i will post a heads-up in the skin changes thread on the forum,
as well as updating the wiki (https://kodi.wiki/view/List_of_Built_In_Controls#DialogSelect.xml)

could you perhaps cherry-pick ronie@5f4435a
which adds this additional button to the Estouchy skin as well.

[DaveTBlake]

Button added to the Estouchy skin as well @ronie

[AlwinEsch]

Good idea, so them can do some extra works via a button about.