flathub: ship desktop file, metainfo and icon from this repository #1581

Part of #1060. Reviewer on com.queryahub.querya_desktop.desktop: "This does not belong here."

Rules (Required metadata): "These metadata files must be integrated in the upstream project." Copies must not be included in the submission PR. Metainfo "must pass validation"; the ID must match exactly; icon preferably SVG or at least 256x256 PNG.

Work

• Add com.queryahub.querya_desktop.desktop, ...metainfo.xml and the icon to this repo (e.g. packaging/linux/flatpak/), used by the Flatpak build and by the other Linux packages
• Metainfo: id, name, summary, description, license (MIT), URLs, categories, OARS content rating, release entries with dates, screenshots (hosted, https), provides for file types
• Prefer a scalable SVG icon (we have the vector logo)
• appstreamcli validate --strict and desktop-file-validate in CI
• Application ID stays com.queryahub.querya_desktop; domain queryahub.com must be verifiable (see QueryaHub/frontend issue)