Kaidan 0.6.0 - stability and some new functions

Kaidan 0.6.0 mostly offers stability fixes and some new smaller features, such as offline messages (three dots appear right when the message has not yet been sent, a check mark appearing as soon as the message has been sent to the server) and a revised correction of messages.

New version features:

- When offline, messages are cached now to be sent later (yugubich)
- It is allowed to correct other messages than the last one now (yugubich)
- Also current messages (untouched) can be corrected now (yugubich)
- Chats can be opened from notifications now (melvo, jbb, peanut)
- A new option to permanently hide the password in Kaidan (melvo)
- New buttons to easily copy the JID and password (jbb, fazevedo)
- Moved pages account administration in settings (jbb)
- The cursor is moved to the end of the text field when you correct a message now (melvo)
- Scanning QR codes without password works now and only JID setting (melvo) results
- The list is called the contact list now (jbb)
- The resource for the displayed presence is selected with fixed rules now (it was accidental before the resource is displayed) (lnj)
- Manage notifications differently on GNOME to keep them in the notification area ( melvo)
- Passed to the implementation of uploading upstream HTTP files (lnj)
- Code refactoring and partial rewriting of the following classes: Kaidan, ClientWorker, RosterManager, PresenceCache, DownloadManager, TransferCache, QrCodeDecoder (lnj, jbb)

The following errors have been fixed:

- Playback issues in previewing media videos (fazevedo)
- Messages sent from other devices are displayed as sent by the chat partner (lnj)
- Notifications are persistently displayed on the screen (jbb)
- List item names are not updated in database (melvo)
- List items are not updated correctly in the model (melvo)
- All sheets contain two headers: Now use the new built-in header property (jbb)
- Buttons that cannot be read with white text on a white background in some styles (jbb)
- Database version not saved correctly (melvo)
- Construction errors with newer versions ZXing (vkrause)

Note: Kaidan requires a C ++ 17 compatible compiler now when building from source.