Fix some Selectors and CSS dialog issues
- fix #142 (closed) #828 (closed) crash on redo
- fix #1168 (closed) crash on undo after closing dialog
- fix #1157 (closed) crash when cloning object
- fix #905 (closed) style tags inside defs ignored
- fix #688 (closed) clear SelectorsDialog when closing document
- fix crash in selectors dialog w/ duplicate window
Disclaimer: These are just incremental improvements, mostly to avoid the crashes. IMHO the dialog needs some significant rewriting, e.g. remove regex parsing of CSS expressions and instead base it more on libcroco.