RSS Feed was not displaying remote images in html description. Closes #1119
HtmlBrowser didn't handle invalid urls

Urls without scheme weren't working (eg //hostname.com/image.png)
When image couldn't be downloaded app would segfault, now stock warning image is displayed instead.
HtmlBrowser loadResource event loop could cause segfault

If HtmlBrowser was updated multiple times from event loop loadResource()
could cause segfault.
HtmlBrowser will use disk cache for resources
HtmlBrowser disk cache will be saved in fsutils::cacheLocation()