View HTML: add CSS helpers with long-press

Move View html code from ReaderHighlight to a new
dedicated module.
Long-press on an element or its text in the HTML will
show a popup with a list of selectors related to this
element that can be copied to clipboard (to be pasted
in Find or in a Book style tweak).
2 addtional buttons in this popup allow seeing all the
CSS rulesets in all stylesheets that would be matched by
this element, which should make it easier understanding
the publisher stylesheets and using or creating style
tweaks.

frontend/document/credocument.lua

@@ -937,6 +937,10 @@ function CreDocument:getHTMLFromXPointers(xp0, xp1, flags, from_root_node)
     end
 end
 
+function CreDocument:getStylesheetsMatchingRulesets(node_dataindex)
+    return self._document:getStylesheetsMatchingRulesets(node_dataindex)
+end
+
 function CreDocument:getNormalizedXPointer(xp)
     -- Returns false when xpointer is not found in the DOM.
     -- When requested DOM version >= getDomVersionWithNormalizedXPointers,