Add SetIcon method on ToolbarAction

[adzo261]

Description:

• Adds SetIcon method on ToolbarAction
• Constructors for ToolbarItems - ToolbarAction, ToolbarSpacer, and ToolbarSeparator now return their concrete type instead of generic ToolbarItem interface. This would allow us to add more methods if needed in future to those structs.

Fixes #2040

Checklist:

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

Example

toolbarIActionItem = widget.NewToolbarAction(theme.FyneLogo(), func() {})
toolbarIActionItem.SetIcon(theme.QuestionIcon())

[Jacalz]

Looks good to me. Thanks for the PR and your first contribution to Fyne :)

[andydotxyz]

Great work, just one thing missed here, sorry!

[andydotxyz]

As this is a new API we need to have a since line. Please add (including the line to separate):

//
// Since: 2.2

The next feature release will probably be 2.2, so we pick this version in anticipation of the next larger release.

[andydotxyz]

Great thanks for this. I will merge it shortly, just wrangling some v2.1.1 fixes first which this needs to come after

[andydotxyz]

Merged on develop, the first feature addition for Bowmore release :)