fix: Properly copy selection as markdown to the plaintext clipboard

[juliushaertl]

Using clipboardTextSerializer we can copy the current selection as markdown and don't need the extra keyboard shortcut, which was broken anyways as it always selected the full text

Steps to reproduce:

• Select a text range
• Copy using Ctrl-C
• Paste to a plaintext editor, github issue or https://evercoder.github.io/clipboard-inspector/

Fix #5213

[max-nextcloud]

Have not tested it yet. Code changes look good. Nice to see more code getting removed than added. :)

I wonder what would happen with non-markdown files. I think the code in the editor factory might serve the purpose of extracting plaintext from the codeblock we currently wrap other files such as source code in.

[max-nextcloud]

Tested and works like a charm.

[juliushaertl]

/backport to stable28

[susnux]

While this works this creates a lot of UX issues, as even if you just copy parts of code it always adds the code tags.
You can no longer e.g. copy email addresses, passwords (you should not do this but thats a different story), shell commands etc.

Could we maybe somehow instead put both into the clipboard? text/plain and text/markdown?