Use Python GTK best practices (#184)

* refactor: rename class `Application` to `DiffuseApplication`
* refactor: rename class `Diffuse` to `DiffuseWindow`
* refactor: remove unnecessary constructor parameters + docstrings
* refactor: move DiffuseWindow class to its own file
* refactor: move PaneHeader/Footer outside DiffuseWindow
* refactor: move FileDiffViewer outside DiffuseWindow
* docs: update CHANGELOG.md

Author: MightyCreak

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Translation: updated Swedish translation (@eson57)
 - Dialog: prompt only once if several files needs to be reloaded (@yuriiz)
 - Use GTK3's Gtk.Application/Gtk.ApplicationWindow (@yuriiz)
+- Split `DiffuseApplication` and `DiffuseWindow` classes in their own files (@MightyCreak)
 
 ### Fixed