refactor(preview_panel): mvc split (#952)

* 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

* fix: set_selection was called with invalid argument

pyproject.toml

@@ -84,7 +84,8 @@ qt_api = "pyside6"
 
 [tool.pyright]
 ignore = [".venv/**"]
-include = ["src/tagstudio/**"]
+include = ["src/tagstudio", "tests"]
+extraPaths = ["src/tagstudio", "tests"]
 reportAny = false
 reportIgnoreCommentWithoutRule = false
 reportImplicitStringConcatenation = false