Deprecate theme.FyneLogo() from theme package

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

We should try to clean up the theme package for icons that we don't need. This Fyne logo was used back in the day but right now it is basically only used in fyne_demo, one single place in the driver as well as in a bunch of tests.

Is it possible to construct a solution with the existing API?

We should remove it, yes.

Describe the solution you'd like to see.

Deprecate the icon and move the tests to using something else. We can bundle the icon in fyne_demo and make sure that the internal driver don't set the logo when no other is provided. It might be better for applications to not have any when the developer does not select one. This makes it more clear that they should add their own icon.

[Jacalz]

This is now solved on develop.