This optional property defines the offset the a key should have
relative to the previous key (on its left) or the start of the
column if it is the first key. If this property is not
present, the key will be placed with no relative offset.
This for example allows keymaps to explicitly define the padding
of the rows that are not "full" relative to other rows, without
guesswork in the code. It is used for this purpose in the
keymaps/levels/rows that needed it.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3162>
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