Add function to put a layer from a different glyph in the background

[mrk-z]

It'd be great to have the function to put a layer from a different glyph in the background.
This is useful when comparing structures between characters, especially for those with similar structures or localizations where we would want certain parts to stay identical.

[justvanrossum]

This relates to #50 (which is essentially about classic background layers), but it sounds like you would prefer a dynamic layer, one that shows an actual live glyph.

This should probably not be a per-source view, but rather it should show the glyph at the current designspace location: you navigate to bold, the reference glyph follows the location of the glyph being edited.

[justvanrossum]

We can add a "reference glyph" field in the text entry panel, where the user can type a character or a glyph name.