additionally small refactoring of UI code

Previously only the "Sign In" event prompted publishing our Data
to the Node. Normally at plugin installation you are already
signed in, so the event doesnt get triggered

The icon can not be recognized. So I used a cropped part of the image.

This is done by overwriting the prepare_context_menu method of the ChatControl
class and was inspired by gajim-otr [1].

[1]: https://github.com/python-otr/gajim-otr/blob/061380/src/gotr/otrmodule.py#L343-L349

Previsouly parts of the logic were in the OmemoButton and OmemoMenu classes.
This commit moves them all into the main Ui class and thus makes it easier to
reuse the functionality in other places. It also extracts some common actions
into their own functions.

* Rework publishing own devicelist

* Dont allow duplicates in received device lists

A bug in a client could lead to a devicelist that has duplicates
in it. We should not transfer these duplicates into our devicelist
of the contact.

Refactor ui because of new popup menu

* Add DB migration to insert 'active' column

Inserts 'active' column into the sessions table with
a Default value of 1

* Set active/inactiv state of sessions

Every device that we receive via devicelist event will be set
to 'active', every device in our DB that is not in the received
devicelist event will be set to 'inactive'

* Show inactive session fingerprints in grey color

Added Min and Max versions, so users with not supported Clients can not activate the Plugin

The UI should be shown when encryption is active in any case
so that the user can disable it.
If for any reason the jid of the contact is not in device_ids
the UI was not shown before. If addtionally the last
known state of encryption was active, the user saw no UI
and couldnt disable omemo. Because for the jid are no devices known
(hence not in device_ids) and omemo = active, because of security
omemo wouldnt let any messages be sent. The user had no
chance to come out of this state

Add attr account to OmemoState. We need this so all current
and future log messages can display the account it is coming from

Remove some log messages which clutter the debug log and
became of low interest because the code works well

An Error was thrown when result was None

Only if at least one Fingerprint is trusted a message goes out
otherwise a error message is printed

cleaned up SQL querys

removed unnecessary return statements

If the UI (Chat Window) is not open a key error is thrown

Arrows could not be displayed in a Windows Log file

- Adds a page where all currently published Device IDs are shown
- Small design changes to the whole Config UI