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This rewrites parts of the main database class. All classes working on
the database have been moved to the new database thread, so inserting
records into the database isn't blocking the user interface anymore.
What also improved the performance *massively* is the use of SQLite
transactions, when inserting multiple records. So inserting is so fast
now that it actually wouldn't necessarily require another thread, but
with this we're safe in the future.

The message model and the roster model have been splitted up into a
database manager and the list model that is used to present cached data
to the user. The XMPP managers are connected to both the model and the
db classes, so both are updated in parallel.

There are also two new classes for the RosterItem and the Message,
because we need to cache those in the models in a vector. The Message
class inherits from QXmppMessage, so we can now use the same class for
sending messages and inserting messages into the database.

Closes #273.

This bring better performances for low power devices and allow better
 and easier filtering from a proxy.

This fix the emoji picker to present the real system emoji that would
 appear in the message view.
There is no reason to present emojis for a font the message view will
 not render identically.

There were several problems with the settings layer (on mobile):
 - You could open the settings layer multiple times
 - When you logged out while being in the settings the settings layer
   didn't disappear

This is only useful on Android and on other platforms supporting this on
the virtual keyboard.

This also allows the user to close the sheet by pressing the 'add
contact' action again.

There are upcoming changes of the expected behaviour when correcting a
message multiple times. The old way was to correct the last correction,
the new way is to always correct the original id.

See https://github.com/xsf/xeps/pull/764

There were three edge cases:
1) The wrong message was replaced:
   - You send a message
   - You open the edit menu
   - Contact or another device of you sends a message
     (lastMessageId is updated)
   - You finish your edit and a message is sent to replace your contacts
     message, which is obviously not what we want. Database and XMPP
     both get the wrong message id to replace.
2) Can't edit last message:
   - You send a message
   - Contact sends a message
     (lastMessageId is updated)
   - You can't edit your last message, although that is expected to work.

The solution was to only update the lastMessageId for own messages.

Also the QML code has been slightly beautified by the use of a signal.

AndroidManifest and click manifest are now changed, when the version is
changed in CMake.

Spoiler messages are used to hide messages containing NSFW content / very long
messages / secret content by default. Kaidan can now send & receive those messages.
A context action on the chat page has been added for this purpose.

Details about the XEP:
https://xmpp.org/extensions/xep-0382.html

Thanks to segal (XMPP Sprint 2019 Berlin) for looking at some of Kaidan's
code and suggesting improvements.

This is a compromise that works acceptable in styles that support colors and those that don't