Support and use modern code for Strikethrough formatting

Seems that in recent times other clients decided to turn to use \x1e as code for strikethrough.

No idea what the guiding document for irc code is, but searchengine seems to favour, and I found related commits e.g. for hexchat:

Given this seems the future, I propose to also switch to sending off formated text using this code, instead of the old.

Tested by chatting between old and new konversation instance, using "%Stest%S".

@buschinski @psn

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