Fixes issue #1104 (Rendering pane flashing)

[buzzert]

For issue #747, a change was introduced that uses private API to disable caching on the preview pane's web view. This was ostensibly to make it so that if images referenced in the markdown document are changed on disk, their changes are reflected in the preview pane.

Unfortunately, perhaps due to some underlying changes to WebKit/WebView, disabling caching will also disable caching for all sorts of other things like stylesheets and the base html document loaded from disk. This results in a momentary flash of empty content if one is typing quickly.

Reverting the changes for #747 resolves the flashing issue, but does regress the issue regarding images being persistently cached.

However, the flashing issue appears to be bothering more people than the image issue, so I believe it would be appropriate to temporarily regress the image caching bug to resolve the flashing bug. Supposedly, a plan exists to migrate to WKWebView, which would hopefully resolve both of these issues simultaneously.

[rogerschlachter]

Would be very good to get this in.

[ClarenceL]

Where's the maintainers? :-(

[aviwad]

Need this.