Support for reactions

[Bios-Marcel]

Current state

Currently reactions can not be seen, added or removed.

Viewing reactions

Viewing reactions should be possible in some simple way.

Currently, I see three viable options:

1. Always showing reactions when a message is selected
2. Toggle it for a single message on selection
3. Toggling it for the whoie chatview with a global shortcut

It would also be possible to implement both 2 and 3 or 1 and 3.

Adding / removing reactions

Adding reactions should be done by selecting the message, hitting some shortcut and entering some kind of "reaction add editor mode". This would be similar to editing a message. But instead you'll see all added reactions as their respective emoji codes. If a new emoji code is added, it will add a new reaction. If you remove one code, it will remove the existing reaction.

Permissions

Since adding reactions requires a certain permission, those have to be checked when adding new reactions. There might be a difference to adding a new reaction and adding one that already has a count of at least one.

[Bios-Marcel]

Commit 42a4104 adds reaction rendering. For now I went with none of the previously mentioned options. Instead, reactions are always visible and can be toggled via the config file.

Still todo:

1. Adding reactions
2. Allowing to see who added a reaction