Calculate widget widths in the WidgetLayoutStore

Note that this ditches all previously set width values, however this is probably acceptable for now. Trying to remain backwards compatible gets tricky on top of already tricky code, and the impact of Element forgetting widths is not as severe as forgetting which widgets were/are pinned.

src/utils/WidgetUtils.ts

@@ -28,7 +28,7 @@ import {WidgetType} from "../widgets/WidgetType";
 import {objectClone} from "./objects";
 import {_t} from "../languageHandler";
 import {Capability, IWidgetData, MatrixCapabilities} from "matrix-widget-api";
-import {IApp} from "../stores/WidgetStore"; // TODO @@
+import {IApp} from "../stores/WidgetStore";
 
 // How long we wait for the state event echo to come back from the server
 // before waitFor[Room/User]Widget rejects its promise

src/utils/numbers.ts

@@ -28,3 +28,7 @@ export function defaultNumber(i: unknown, def: number): number {
 export function clamp(i: number, min: number, max: number): number {
     return Math.min(Math.max(i, min), max);
 }
+
+export function sum(...i: number[]): number {
+    return [...i].reduce((p, c) => c + p, 0);
+}