T.me link handling

[alvitg]

Please enable proper handling of links like https://t.me/durov_russia/29 Just now clicking such a link in Fernshriber tends to USERNAME_INVALID message.

[jgibbon]

While it's not really documented in a traditional way, a look at the Telegram Desktop source will help find some more examples:
const std::vector<LocalUrlHandler> &LocalUrlHandlers() (and the methods it calls) in
https://github.com/telegramdesktop/tdesktop/blob/dev/Telegram/SourceFiles/core/local_url_handlers.cpp give kind of an overview.

[jgibbon]

This seems to belong to a bunch of related issues: #151, #276, #312, #412 (this comment is copied to all of them)
Proposed way to hopefully fix most of them:

• use MimeType=x-url-handler (caveat: according to link, update-desktop-database isn't called after installation yet…)
• implement at least some of the missing url functions (link in T.me link handling #312 – search for LocalUrlHandlers; also look below)
• on start: scan for existing .desktop file, delete.
• link handler: save "active link" to some temporary property until tg is authenticated, only then process it.