[BUGFIX] Use non-JavaScript based iframe src

[andreaskienast]

Since the release of Chrome 73, iframe documents can't be loaded via
javascript protocol anymore. This commit changes the iframe src to
be an empty page for WebKit browsers.

Many thanks go to @guppy42 who identified the cause and proposed this fix.

Resolves: #26

[guppy42]

Unfortunately while this fixes Chrome it seems it now breaks safari instead - due to about:blank; being seen as cross origin.

To fix this turns the line into a bit of soup;

src: UserAgent.isGecko ? 'javascript:void(0);' : (UserAgent.isWebKit ? (UserAgent.isChrome ? 'about:blank;' : 'javascript: \'' + Util.htmlEncode(this.config.documentType + this.config.blankDocument) + '\'' ): HTMLArea.editorUrl + 'Resources/Public/Html/blank.html')

It's ugly but works - undoubtedly there is a more elegant solution