Vue initialization and FloatingMenu codebase refactoring and cleanup (#649)

* Clean up Vue initialization-related code

* Rename folder: dispatcher -> interop

* Rename folder: state -> providers

* Comments and clarification

* Rename JS dispatcher to subscription router

* Assorted cleanup and renaming

* Rename: js-messages.ts -> messages.ts

* Comments

* Remove unused Vue component injects

* Clean up coming soon and add warning about freezing the app

* Further cleanup

* Dangerous changes

* Simplify App.vue code

* Move more disparate init code from components into managers

* Rename folder: providers -> state-providers

* Other

* Move Document panel options bar separator to backend

* Add destructors to managers to fix HMR

* Comments and code style

* Rename variable: font -> font_file_url

* Fix async font loading; refactor janky floating menu openness and min-width measurement; fix Vetur errors

* Fix misaligned canvas in viewport until panning on page (re)load

* Add Vue bidirectional props documentation

* More folder renaming for better terminology; add some documentation

frontend/src/utility-functions/files.ts

@@ -0,0 +1,46 @@
+export function downloadBlob(url: string, filename: string): void {
+	const element = document.createElement("a");
+
+	element.href = url;
+	element.setAttribute("download", filename);
+	element.style.display = "none";
+
+	element.click();
+}
+
+export function download(filename: string, fileData: string): void {
+	const type = filename.endsWith(".svg") ? "image/svg+xml;charset=utf-8" : "text/plain;charset=utf-8";
+	const blob = new Blob([fileData], { type });
+	const url = URL.createObjectURL(blob);
+
+	downloadBlob(url, filename);
+
+	URL.revokeObjectURL(url);
+}
+
+export async function upload(acceptedEextensions: string): Promise<{ filename: string; content: string }> {
+	return new Promise<{ filename: string; content: string }>((resolve, _) => {
+		const element = document.createElement("input");
+		element.type = "file";
+		element.style.display = "none";
+		element.accept = acceptedEextensions;
+
+		element.addEventListener(
+			"change",
+			async () => {
+				if (element.files?.length) {
+					const file = element.files[0];
+					const filename = file.name;
+					const content = await file.text();
+
+					resolve({ filename, content });
+				}
+			},
+			{ capture: false, once: true }
+		);
+
+		element.click();
+
+		// Once `element` goes out of scope, it has no references so it gets garbage collected along with its event listener, so `removeEventListener` is not needed
+	});
+}