[Edge] Add fallback when WebView2 runtime is not present eclipse-platform#2000

When using WebView2 as browser engine in SWT without a WebView2 runtime
being available on the system, browser initialization fails. In order to
more gracefully handle the case that a system has no such runtime
installed (like on some Windows 10 systems), this change introduces an
automatic fallback to Internet Explorer in case no WebView2 runtime is
found. In addition, it shows an error dialog informing about the missing
runtime and showing a link to FAQ about it.

Fixes eclipse-platform#2000

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT Browser/common/org/eclipse/swt/browser/Browser.java

@@ -93,10 +93,7 @@ public Browser (Composite parent, int style) {
 	}
 
 	style = getStyle ();
-	webBrowser = new BrowserFactory ().createWebBrowser (style);
-	if (webBrowser != null) {
-		webBrowser.setBrowser (this);
-		webBrowser.create (parent, style);
+	if (createWebBrowser(parent, style)) {
 		return;
 	}
 	dispose ();
@@ -119,6 +116,35 @@ public Browser (Composite parent, int style) {
 	SWT.error (SWT.ERROR_NO_HANDLES, null, errMsg);
 }
 
+private boolean createWebBrowser(Composite parent, int style) {
+	webBrowser = new BrowserFactory ().createWebBrowser (style);
+	if (webBrowser == null) {
+		return false;
+	}
+	webBrowser.setBrowser (this);
+	try {
+		webBrowser.create (parent, style);
+		return true;
+	} catch (SWTError error) {
+		boolean isEdge = "win32".equals(SWT.getPlatform()) && (style & SWT.IE) == 0;
+		if (isEdge && error.code == SWT.ERROR_NOT_IMPLEMENTED) {
+			return fallBackToIE(parent, style);
+		}
+		throw error;
+	}
+}
+
+private boolean fallBackToIE(Composite parent, int style) {
+	getDisplay().asyncExec(() -> {
+		MessageBox fallbackInto = new MessageBox(getShell(), SWT.OK | SWT.ICON_ERROR);
+		fallbackInto.setText("WebView2 runtime not available");
+		fallbackInto.setMessage("Falling back to Internet Explorer for browser widgets. For information on how to install WebView2, see https://github.com/eclipse-platform/eclipse.platform/tree/master/docs/FAQ/FAQ_How_do_I_use_Edge-IE_as_the_Browser's_underlying_renderer.md");
+		fallbackInto.open();
+	});
+	System.setProperty(PROPERTY_DEFAULTTYPE, "ie");
+	return createWebBrowser(parent, (style | SWT.IE) & ~SWT.EDGE);
+}
+
 static Composite checkParent (Composite parent) {
 	String platform = SWT.getPlatform ();
 	if (!"gtk".equals (platform)) return parent; //$NON-NLS-1$