Never copy computed style

Modify how the style attribute is copied and (with "paste style") pasted to never recompute the style. Also modify "paste style" to overwrite the class attribute as well. With these changes, copying and pasting objects will no longer modify the style attribute, and "paste style" now overwrites the target's style and class attributes with the verbatim values of the object on the clipboard. (Note that this includes removing said attributes from the target if they are not set on the source.)

This, due to CSS rules, may result in "paste style" producing a different effective style on the target than on the source, but is arguably more correct. It may also cause problems when copying and pasting across documents, though users that would be affected (i.e. due to use of CSS styling) are likely to be aware of the cause.

Fixes #1153 (closed), #2594 and inbox#5130 (closed).