theme: Move icons to app folders
- This improves modularity. Each app brings its own icons in its directory instead of a centralized directory.
Tests:
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Install an app and notice that the installation notification has the icon.
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Visit an app's page in Apps and System (cockpit) section and ensure that the app's icon is being shown.
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Visit the Apps and System section and notice that apps' icons are being shown.
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Visit the home page and notice that shortcuts' icons are being shown.
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Visit the URL /plinth/api/0/shortcuts and notice that the URLs for icons are accessible and lead to proper icon files.
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Build developer documentation and notice that Tutorial -> View and Reference > Menu pages show the expected updates.
Signed-off-by: Sunil Mohan Adapa sunil@medhas.org