Add print cycle events and loading msg

Since the print cycle now loads all images not-yet-loaded (due to lazy loading), there can be a moment of time where it appears pressing Get PDF is doing nothing, depending on connection speed.  To add a "loading" message, a custom event is fired at the start and end of the print cycle (before the print dialog comes up).

client/homebrew/navbar/print.navitem.jsx

@@ -1,9 +1,25 @@
-import React from 'react';
+import React, { useState, useEffect } from 'react';
 import Nav from './nav.jsx';
 import { printCurrentBrew } from '@shared/helpers.js';
 
 export default function(){
+	const [printing, setPrinting] = useState(false);
+
+	// listen for print cycle events to display "loading" message since it can take some time.
+	useEffect(()=>{
+		document.addEventListener('print:startprep', handlePrintStartPrep);
+		document.addEventListener('print:finishedprep', handlePrintPrepFinished);
+		return ()=>{
+			document.removeEventListener('print:startprep', handlePrintStartPrep);
+			document.removeEventListener('print:finishedprep', handlePrintPrepFinished);
+		}
+	}, []);
+
+	const handlePrintStartPrep = ()=>{ setPrinting(true); };
+
+	const handlePrintPrepFinished = ()=>{ setPrinting(false);	};
+
 	return <Nav.item onClick={printCurrentBrew} color='purple' icon='far fa-file-pdf'>
-		get PDF
+		{printing ? 'loading' : 'get PDF'}
 	</Nav.item>;
 };