History

Log history into a file. Currently without encryption.
Notable changes:
1. ChatAction's setup(...) function doesn't clear stored content in the class instance after call (for MessageAction, SystemMessageAction, etc). It is much easier to operate with chat area using this class' instance (past history, for example) in cost of memory use.
2. Clear chat area (like in PR #420) with correct unfinished downloads handling but without shortcuts.

Things left to do:
1. Finish infrastructure for saving history as an encrypted file.
2. Implement correct actions saving.
3. Don't mix history from different profiles (when associated PR will be accepted).
4. Groupchats history: as for now it is no way to distinguish one group chat from another. I mean chat names or IDs are not persistent (probably won't be a problem after "New Group Chats" introduction).
5. Tests, tests, tests :)