Feature/canvas focus traversal

[toaster]

Description:

This PR adds the functions FocusNext() and FocusPrevious() which already exist in the glCanvas as well as the testCanvas to the fyne.Canvas interface.
This allows to traverse through the focusable items programmatically and also eases the testing of the upcoming menu keyboard control feature.

In addition it adds focus managers to mobile canvas which reduces the lag that is addressed by #1625.

Checklist:

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

Where applicable:

• Public APIs match existing style.