v1.6.0 — SVG icons + Firefox data-collection declaration

Replaces emoji/text-only labels throughout the popup and the on-page draw/highlight toolbars with real SVG icons (source set in design/icons/); redrawn toolbar icons to match. Adds browser_specific_settings.gecko.data_collection_permissions to the manifest (required: none, optional: websiteContent) — Mozilla now requires this before AMO will accept a new extension for signing.