Add support for ico file when compiling for windows

[zivoy]

Is your feature request related to a problem? Please describe:

It is currently not possible to use a .ico file as an icon for the app using the fyne.Window.SetIcon function, though this might be a bit harder to do on OS's that are not Windows.
BUT when compiling for windows you also can't use a .ico file, it tries to make a .ico file from a format it can read (png)

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

You can use a png as the app icon and as the desktop icon

Describe the solution you'd like to see:

At least when compiling for Windows and you supply a .ico file it should use it instead of trying to make a new one

Note:

This was discussed here on the Discord Gophers discord

[andydotxyz]

This issue mentions a couple of different items.
It was discussed to day that:

• Fyne (and fyne-cross too) should support packaging with a windows icon
• SetIcon does not have to process windows icon files as it uses Go image decoders, and all images should be cross-platform formats, or a custom image decoder loaded in your app.

So we will progress with fixing the packaging issue.

[andydotxyz]

On develop for v2.2.0