chore(lib): re-organizing the module

This PR refactors a lot of the code in order to have a better organized codebase. If will that I needed to create a second level of submodules, to better distinguish the different parts of this project.

manim_slides/cli/qt_utils.py

@@ -0,0 +1,14 @@
+"""Qt utils."""
+
+from qtpy.QtWidgets import QApplication
+
+
+def qapp() -> QApplication:
+    """
+    Return a QApplication instance, creating one
+    if needed.
+    """
+    if app := QApplication.instance():
+        return app
+
+    return QApplication([])