gui: remove document.execCommand calls

gui: remove document.execCommand calls

The `document.execCommand` call is deprecated since a few years [0] so I
went ahead and removed it. We only use it to copy stuff to the clipboard
and the recommended way now is to use `navigator.clipboard.writeText`
[1]. `writeText` is kind of new, but I think we'll be alright regarding
compatibility (Compat table is also available at [1]).

Making the handler functions async is okay because extjs executes the
handler and does not expect any result from it, nor does it need to do
some work afterwards.

[0]: https://developer.mozilla.org/en-US/docs/Web/API/document/execCommand
[1]: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard/writeText

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>