Add desktop applications to Steam

[Janleyx]

What changed

• Add an Add to Steam helper for desktop files, executables, AppImages, scripts, and Windows executables.
• Add the action to Dolphin/KIO context menus.
• Add the action to Plasma’s application launcher.

Why

Armada users currently have to add desktop applications through Steam manually. This provides the SteamOS-style desktop integration as an independent feature that can be reviewed and tested separately from Nested Desktop.

Validation

• bash -n system_files/usr/bin/steamos-add-to-steam
• git diff --check
• Full Armada shell test suite passed on the current upstream base.
• The KDE service-menu files use KDE-specific Type=Service extensions, which the generic freedesktop desktop-file validator does not recognize.

[Janleyx]

Apologies for the long delay here. I have been very busy lately, but I plan to revisit this patch soon, update it against the latest Armada release, and revalidate it on-device before requesting another review. Thank you for your patience.