Refactor verbatim copy style into separate actions

After discussing the recently merged feature to allow copying and pasting the verbatim class and style attributes, rather than the default behavior of copying and pasting the computed style, it was decided that we prefer to have both options available as separate actions. Accordingly, split the existing "paste style" into "paste computed style" and "paste verbatim style", adjusting the guts as needed.

Note that this adds an "inkscape:computed-style" attribute to our "additional information" node when copying to the clipboard, which contains the computed style. This information was previously saved in the "style" attribute, which now contains a verbatim copy of the "style" attribute of the source object instead. This may result in unexpected behavior if users are somehow copying and pasting between different versions of Inkscape.