Add support for `{{clipboard}}` placeholder for clipboard content

[Shaniser]

Description

It would be incredibly useful to introduce a {{clipboard}} placeholder that dynamically replaces itself with the current content of the clipboard when used in menu item URIs / commands / paste.

Affected Desktop Environments

All

Additional Information

For example, it can be used by developers to open specific task in Jira or for pasting repeatable template.

[Schneegans]

Yeah, that's a cool idea. Maybe this could be implemented alongside #573.