RFC: Introduce a new Edge container

[Jacalz]

Description:

This new container is a replacement for container.NewBorder but without the footguns. There is a centre item instead of a slice of objects that act like a Stack container. The centre item can optionally be nil also.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.

[Jacalz]

This uses the layout.NewEdge name which stutters less than layout.NewEdgeLayout but is less similar to the other layouts. Should we rename (I.e. create new functions and deprecate the old ones) with better naming?

[coveralls]

coverage: 65.017% (+0.04%) from 64.975%
when pulling 418cfb3 on Jacalz:edge-container
into 56342a2 on fyne-io:develop.

[andydotxyz]

Hmm, if we think that having no 5th parameter for border we could potentially move it from 4 to 5 parameters and the optional becomes 6 onwards in case people did use the ability to have more than a single item in the centre.

I'm not totally sure it's worth a new layout (and new container?) to fix this one issue.

[Jacalz]

yeah, probably not