Add transparent overlay window

[Nerixyz]

Description

This PR adds a hotkey to open a transparent overlay channel-view. The colors of the overlay can be customized in the theme. To move, resize, and close the overlay, you have to press the SHIFT key.

TODO

This PR isn't fully done yet. Feedback on the implementation is welcome.

• Wait for Use Qt's High-DPI scaling on Windows #4868
  This is the main blocker right now, but there's no point in merging yet another thing that (possibly) interacts poorly with high-DPI.
  • Depends on Font/UI scaling improvements #3690
• Add dialog explaining functionality after first time opening.
• Test /watching.
• Fix split lifetime (Allow ChannelViews without a Split #4747)
• Check theme colors
• Fix cursor
• Add minimize button
  • Maybe use NCHITTEST?
• Draw background behind buttons when SHIFT is held.
• Support click-through
• Support multiple click-through windows
• Add [optional] shadow (maybe future PR?)
• High DPI tests
• Update theme migrations (wiki)
• Support scaling (independent - see also: Add the ability to control the popup chat font size separately. #1844)
• Check margins (and interaction overlay underlay)
• Add documentation
• Maybe some transparency changes can be in a separate PR?
• Support blur behind/acrylic/... [mica?] → In another PR - https://github.com/tauri-apps/window-vibrancy/ provides a good foundation

firefox_2023-08-02_19-57-18.mp4

Link to the video in case the embed is broken: https://github.com/Chatterino/chatterino2/assets/19953266/3e798169-a82f-4123-b834-7ce55daf0fc1

Closes #1618.

[RangerFrank]

I've been using https://github.com/baffler/Transparent-Twitch-Chat-Overlay for the functionality that this PR is going to add to Chatterino, very glad to see it finally happening. One major feature that is an oversight for many chat overlays is that it needs to have toggleable click-through, so that the widget can be sized and moved, but when toggled won't be able to be clicked or interfere with mouse pointer tracking in games and other programs.