Rather than a drop down dialog, which could present users with images
that may not run on their hardware, allow selection of Raspberry Pi as a
first stage. If users adopt this feature, they are presented with a
subset of images that we know will actually run on their hardware.
This is achieved by leveraging @maxnet's excellent OS filtering scheme.
Future work will attach image and description support to this OS list.
If you load a custom image by running `rpi-imager my_image.img.xz` then
the Advanced Options button is not shown. This is triggered if you set
a file association to load rpi-imager and then double-click an image
file.
The fix is simply to set the visibility on this button on start-up
(custom images set on the command-line are loaded before this).
Fixes#451
