WinForms: Prevent designer from setting default values

[merceyz]

This PR solves the issue described in #2206 (comment)

The changes in this PR prevents the designer from setting the default value of specific properties which caused it to attempt to load CefSharp.Core causing an exception to be thrown

[AppVeyorBot]

❌ Build CefSharp 63.0.0-CI2484 failed (commit 760eca7999 by @merceyz)

[amaitland]

Should we do the same with other properties like BrowserSettings?

[merceyz]

We could but we don't have to. These were the only ones causing a issue.

If you add a control to a form the designer adds this code, so these would be the ones to add it to

this.chromiumWebBrowser1.BrowserSettings = null;
this.chromiumWebBrowser1.DialogHandler = null;
this.chromiumWebBrowser1.DisplayHandler = null;
this.chromiumWebBrowser1.DownloadHandler = null;
this.chromiumWebBrowser1.DragHandler = null;
this.chromiumWebBrowser1.FindHandler = null;
this.chromiumWebBrowser1.FocusHandler = null;
this.chromiumWebBrowser1.GeolocationHandler = null;
this.chromiumWebBrowser1.IsActivating = false;
this.chromiumWebBrowser1.JsDialogHandler = null;
this.chromiumWebBrowser1.KeyboardHandler = null;
this.chromiumWebBrowser1.LifeSpanHandler = null;
this.chromiumWebBrowser1.LoadHandler = null;
this.chromiumWebBrowser1.MenuHandler = null;