Show offset in the layer list

[Polarbear-j]

Imagery offset highly depends on the area, even for the same layer. Thus when I move to another area, I need to get reminded that there is an offset still set to the old area.

Now (13064) I have to right-click the layer, go to offset and check what is set. I propose to show the current offset as a column in the layer list, maybe even coloured when not zero. Could be either the name or the numeric value, the latter is a bit more predictable in length.

In this context, I propose a simpler method to set the offset to zero. Currently I either have to delete and reload the layer, or to create a "zero" offset setting. Either allow to un-tick an existing setting, or show a zero-button in the submenu.

[Klumbumbus]

see also #15711 and #12173

[Klumbumbus]

Closed as duplicate of #15711.
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