refactor(preview_thumb): mvc split (#978)

* refactor: basic split

* fix: renaming and usage test didn't work for the tests

* fix: tests

* refactor: restructuring

* refactor: further separation and lots of related changes

* refactor: remove last reference to a widget from controller

* refactor: address todo

* fix: failing tests and mypy compaint

* refactor: move control logic to controller

* refactor: more readable button style

* refactor: move existing code to view

* refactor: move existing code to controller

* fix: imports

* refactor: make methods private by default

* refactor: privatise fields

* refactor: reduce code duplication

* refactor: consolidate and sort display methods

* refactor: remove needless setting of delete action text

* refactor: extract control logic from _display_file

* refactor: use MediaType for __switch_preview

* fix: import in preview_panel_view.py

* refactor: remove unnecessary wrapper on view side

* refactor: move image data retrieval to control side

* refactor: move audio / video specific code to the respective method

* refactor: remove superfluos methods

* refactor: this and that

* refactor: use proper type instead of dict for file stats

* refactor: extract gif parsing to controller

* refactor: extract video size extraction to controller

* doc: add rule of thumb to Qt MVC Style Guide

* doc: change rule of thumb from note to tip

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Co-authored-by: Travis Abendshien <46939827+CyanVoxel@users.noreply.github.com>

STYLE.md

@@ -78,5 +78,7 @@ Observe the following key aspects of this example:
   - Defines the interface the callbacks
   - Enforces that UI events be handled
 
+> [!TIP]
+> A good (non-exhaustive) rule of thumb is: If it requires a non-UI import, then it doesn't belong in the `*_view.py` file.
 
 [^1]: For an explanation of the Model-View-Controller (MVC) Model, checkout this article: [MVC Framework Introduction](https://www.geeksforgeeks.org/mvc-framework-introduction/).