plymouth: support question and message widgets

[0x5a4]

I've added support for the plymouth question and message widgets (shown via plymouth ask-question and plymouth display-message respectively). These can by pretty useful for more sophisticated setups that require more than "please enter a password".

You can try them out via

boot.initrd.systemd.services.plymouth-start.postStart = ''
  ${lib.getExe' pkgs.plymouth "plymouth"} display-message --text "Hi! I am a Message"
  ${lib.getExe' pkgs.plymouth "plymouth"} ask-question --prompt "Answer me!"
'';

Please let me know what you think about the placement of the message. I'm a bit unsure where to put it...

[danth]

What about placing the message at baseline_y? (Without shifting it by the height of the image.) That might look consistent with the other prompts.

[0x5a4]

The issue then is that we get something like the following, because message isnt exclusive.

Enter password
Hi I am a Message
••••••••••••

I've found this to be pretty confusing...

While we're at it: What do you think about merging the two theme files? This could easily be achieved by inlining the theme into the module and just use regular nix conditionals.
Also what about an option to not show the password bullets?
(I'd be willing to test + implement both)

[0x5a4]

How about

Hi I am a Message!

Enter Password

•••••••

though I think we'd have to cheat a bit, because prompt may not be there. but we could get there using a dummy label (which is quite ugly)

[trueNAHO]

On a sidenote, I am unsure whether I like Stylix applying such arbitrary configurations instead of end-users implementing this themselves by accessing Stylix variables. However, I see the convenience since most people might not bother configuring their splash screen.

[0x5a4]

Plymouth also has the most horrible documentation i've ever seen... You are forced to just copy paste from others and adapt, which I find to just not be very enjoyable.