Refactor the whole structure, no more separation for C++ and Slint files

ui/windows/AddSourceWindow.slint

@@ -1,41 +0,0 @@
-import { Backend } from "../Backend.slint";
-import { VerticalBox, CheckBox } from "std-widgets.slint";
-import { SideBar } from "../components/SideBar.slint";
-import { VButton, VText, VTextInput, Palette } from "@selenite";
-
-export component AddSourceWindow inherits Window {
-
-	title: "Mirai - Add source";
-	min-height: 100px;
-	max-height: 4000px; // needed, otherwise the window wants to fit the content (on Swaywm)
-	min-width: 400px;
-	default-font-size: 16px;
-	background: Palette.background;
-
-	in-out property <string> default-source-path;
-
-    VerticalLayout {
-		padding: 16px;
-		spacing: 8px;
-		nameInput := VTextInput {
-			label: "Name";
-			text: "todo";
-		}
-		pathInput := VTextInput {
-			label: "Path";
-			text: root.default-source-path + nameInput.text + ".md";
-		}
-		VButton {
-			text: "Create";
-			clicked => { 
-				Backend.add-source({
-					name: nameInput.text,
-					type: "FileSystemMarkdown",
-					path: pathInput.text
-		  		})
-			}
-		}
-     }
-}
-
-export { Backend, Palette } // Export to make it visible to the C++ backend