[msbuild] Add support for alternate app icons.

[rolfbjarne]

This adds support for including all app icons in the app, as well as for specifying alternate app icons.

Including all app icons is enabled by setting the following MSBuild property:

<PropertyGroup>
    <IncludeAllAppIcons>true</IncludeAllAppIcons>
</PropertyGroup>

This will make the ACTool task pass --include-all-app-icons to actool when compiling the asset catalog.

Specifying alternate app icon(s) is enabled by setting the following MSBuild item:

<ItemGroup>
    <AlternateAppIcon Include="MyAlternateAppIcon" />
    <AlternateAppIcon Include="MyOtherAlternateAppIcon" />
</ItemGroup>

Note that the included value is the name if the icon in question, not the path or the filename of the actual icon file (this way there's no need to have different items for different platforms, because there are some platform differences with regards to paths and icon file extensions).

In order to make the behavior consistent, this PR also adds support for specifying the app icon itself using an MSBuild property (in addition to the existing Info.plist property XSAppIconAssets, which will eventually be deprecated):

<PropertyGroup>
    <AppIcon>MyAppIcon</AppIcon>
</PropertyGroup>

The same note applies here: the value is the name of the icon.

Note about platforms: the code is there for all platforms (iOS, tvOS, macOS and Mac Catalyst), but alternate app icons only seem to work for iOS and tvOS. UIApplication.SupportsAlternateIcons always returns false on Mac Catalyst (googling only find people in the same situation), and macOS doesn't have this API to begin with (nor anything similar):

Also:

• Add a bunch of tests.
• Document on our website: https://github.com/dotnet/docs-mobile/pull/53
• Samples for iOS and tvOS: [AlternateAppIcons] Add new sample for specifying and using alternate app icons on iOS and tvOS. dotnet/macios-samples#2

Fixes #12790.
Fixes #20182.

[rolfbjarne]

CC @Redth since you requested this feature in #12790.

[github-actions]

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[github-actions]

⚠️ Your code has been reformatted. ⚠️

If this is not desired, add the actions-disable-autoformat label, and revert the reformatting commit.

If files unrelated to your change were modified, try reverting the reformatting commit + merging with the target branch (and push those changes).

[vs-mobiletools-engineering-service2]

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[vs-mobiletools-engineering-service2]

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[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

✅ API diff vs stable

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

ℹ️ Generator diff

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[vs-mobiletools-engineering-service2]

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[vs-mobiletools-engineering-service2]

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[vs-mobiletools-engineering-service2]

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✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
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✅ monotouch (MacCatalyst): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 9 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 7 tests passed. Html Report (VSDrops) Download
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[dalexsoto]

re 👍