Send button as alternative to enter key

[SpiritCroc]

One user requested a send button next to the message input field, which can be used instead of the enter button to send messages.
The use-case is likely when having the setting to use enter-button to create newlines (-> Settings/preferences/Use Ctrl + Enter to send a message - element does not have this yet btw, it was picked from here), and not having to remember how to send messages in this case.
The icon is probably best taken from SchildiChat-Android, and we might want to make this a setting (off by default?) - or maybe show it only when ctrl+enter to send is enabled.

[su-ex]

Introduced upstream in v1.7.23