WIP: Slideout preview

[weisJ]

Note: This builds on top of #1206. The relevant commits are the newest two.

Implements a basic version of the new preview concept proposed in #819 (comment)

Things up for discussion, which mostly aren't implemented yet:

• Should the header span the whole width or only be on the left column?
• Should Maccy remember the open/close state of the preview sidebar across being opened?
• The blog post in the linked comment implements auto closing/opening the sidebar based on the width. Is this desirable?

Things that don't properly work yet:

• There is a minimum width on the history list (or a maximum width on the preview) preventing the sidebar to be resized larger than a certain value (around 400px).
• No toolbar sidebar toggle button yet.
• Translations for the settings are missing
• If Maccy is opened to far on the right of the screen (such that the sidebar doesn't have enough space) then the sidebar should open on the left. The opening animation and layout for this is already working, just the detection of which sidebar placement to choose doesn't work yet.
• Somehow resizing the window is a bit tricky. Not sure what the reason is.

Demo video:

Screen.Recording.2025-12-27.at.11.28.46.mov