Non-blocking title and description fetching

[kirillt]

The app must load missing title or description in background. The link without title or description still should be added into the list, but spinning circles should be used to denote missing fields.

About storage, if the user provided:

• no title and no description — we store .link file, but we don't store anything in user/properties
• only description — besides .link file, we store JSON with single field description
• only title — same, we create JSON with single title field
• both — we create JSON with both

The cache/metadata storage should be used to store automatically fetched title and description, this is as fallback for user/properties (when 1 or 2 fields are missing). If a title or description is missing in both properties and cache, then we fetch them from the web. If there is no internet connectivity, then spinning circle can be replaced by dedicated "offline" icon.

[kirillt]

This also means that .link files copied externally into the folder must be treated same as those which were added through the app. E.g. there are 3 .link files in the root folder, all with properties attached. The user copies 2 other .link files from other folder. The app must display 5 links and request title and description for 2 of them.