STORM-34 ADDITIONAL_FIX Implemented storing of multi-user favorites and showing them on login screen.

- Changed the way SLURLs are cached a little, because previous one introduced problems with theit order.

- Also allowed saving of favorites to disk even if not all of them received SLURL info - this is done to avoid favorites not saving when there is at least one "dead" landmark among them.

- "Username" field on login screen is now not a lineeditor, but combobox (to enable autocompletion), but without button (Esbee asked for this in ticket for security reasons, and perhaps for visual consistency).

- Elements of this combobox are names of users whose favorites we have saved in file.

- Contents of "Start at:" combobox are changed depending on changes in "Username"- if username is present in favorites file, favorites for this user are added there.

- New callback was added to LLCombobox and used in this fix, because present ones weren't enough to easily track changes in text entry.