Add save page option

[jetownfeve21]

open a file dialog to choose the path of the file which is used by the
save() method. There is no css and images.

fix #77

[mgautierfr]

You are using the flag QWebEngineDownloadItem::CompleteHtmlSaveFormat when you save the page and the QT documentation says that it will save the resources.
Does the commit's comment ("There is no css and images.") is still valid ?

[mgautierfr]

I think that the main problem with the images and the resources is that we use the zim:// as url scheme to intercept the request and serve the content from the zim file. (And forward the request to the native browser if this is something else than a zim://... request).
The save mechanism of Qt must be lost with this custom scheme and don't save the images and other resources.

[mgautierfr]

And this PR implement the save action from the "top bar" menu. But the "Save page" from the popup menu still does nothing.

[jetownfeve21]

@kelson42
Do I keep working on it ?

[kelson42]

I think this PR needs to be closed without been merged. Too complicated for now to get it work properly.