[Settings][Run] Improve XAML and settings page loading

[davidegiacometti]

Summary of the Pull Request

• Use ItemTemplateSelector for plugin additional options
• Replace ItemsControl with ItemsRepeater for plugins list to improve the loading time of the page

PR Checklist

• [x] Closes: #32015
• [ ] Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• [ ] Tests: Added/updated and all pass
• [ ] Localization: All end user facing strings can be localized
• [ ] Dev docs: Added/updated
• [ ] New binaries: Added on the required places
  • [ ] JSON for signing for new binaries
  • [ ] WXS for installer for new binaries and localization folder
  • [ ] YML for CI pipeline for new test projects
  • [ ] YML for signed pipeline
• [ ] Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Introducing templates for plugin options didn't really improve page load time so I investigated a little bit. I profiled the page loading using ItemsControl and ItemsRepeater (the second one uses virtualization). Here is the comparison with a Debug build and 24 plugins loaded:

The loading speed improvement it's clearly visible.

Validation Steps Performed

Manually tested Run settings page:

• Verified that the page loads faster
• Verified that all plugins are visible
• Verified that all plugin options are visible

[Jay-o-Way]

Any graphic changes? (looking at <Rectangle Style="{StaticResource OptionSeparator}" />)

[davidegiacometti]

Any graphic changes? (looking at <Rectangle Style="{StaticResource OptionSeparator}" />)

No visual changes. This is https://github.com/microsoft/PowerToys/blob/41a0114efe903b268b5a2ed27a33b423aa9910ea/src/settings-ui/Settings.UI/SettingsXAML/Views/PowerLauncherPage.xaml#L644-L648

[Jay-o-Way]

I was thinking DividerStrokeColorDefaultBrush coulda be nice as a separator too.