Drop-down cosmic terminal fixed

[rnbwdsh]

• Add --drop-down CLI flag, that adds new behavior and keeps all the old behavior if not used
• Behavior on first launch: create a surface, add a signal handler, render the terminal into that overlay surface
• Behavior on later launches: find the process that was launched with "--drop-down" and send a signal to it (currently via spawning a process)
• Handle USR1 signals for toggling dropdown
• Custom right click context menu in dropdown mode, as maximizing makes no sense
• Added setting: A secondary show-headers-dropdown that's only visible in drop down mode
• Added setting: The height of the dropdown in px, I'd like to do relative but to my understanding wayland doesn't trivially allow that
• Bug byepass in iced: When creating the overlay without any delay, the window is transparent but not "frosted"

This is hugely based on pop-os#673 and just rebased and refactored, as various menu things were broken for me.

I think this MR is relevant, because people are already vibe coding a non-working version of this: https://github.com/M0Rf30/cosmic-ext-quake-terminal

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