Show the app icon of appdata files loaded from local

[ryonakano]

We can load and show the local appdata file with the --load-local option, but the app icon is not loaded and the application-default-icon is used instead.
I guess it would be useful e.g. when we redesign an app icon and want to check how it looks like in the AppCenter (like the contrast between the icon and the background color of the banner) if we can also specify and load a local app icon.

[davidmhewitt]

There are multiple things about this that are problematic.

• Firstly, icons are not usually specified in AppData manually, they're usually added at the build stage by appstream-generator so we'd have to think about how to specifiy which icon to load
• Secondly, when real icons are loaded in AppCenter, they come from a rasterized source (i.e. appstream-generator takes the nice svg you've made and renders it to a png to ship in the repo). We've seen multiple cases of this rendering going a bit wrong so if we loaded an svg here, it wouldn't necessarily be representative of the final result and may give a false impression of the possible fidelity/quality of the final icon.

While I agree it would be a useful feature to have, the technical constraints mean it's semi-difficult to implement and may not give the couple of developers that use the feature a representative result anyway.

[aral]

Workaround: You can preview your app icon with --load-local if you specify its identifier in your appdata.xml.in file in an icon tag with type set to stock.

For example:

<icon type="stock">org.small_tech.comet</icon>

(Just make sure to comment it out when you’re done testing or your Flatpak builds will fail.)