Add a loading toast when update download begins

interface.d.ts

@@ -63,6 +63,9 @@ export interface IElectronAPI {
   kittycad: (access: string, args: any) => any
   listMachines: () => Promise<MachinesListing>
   getMachineApiIp: () => Promise<string | null>
+  onUpdateDownloadStart: (
+    callback: (value: { version: string }) => void
+  ) => Electron.IpcRenderer
   onUpdateDownloaded: (
     callback: (value: string) => void
   ) => Electron.IpcRenderer