ctools_css_disassemble() sometimes overwrites a selector

Patch attached checks whether the $selector_str variable already exists or not in the $disassembled_css array. It makes sure the declaration of a selector is merged into one array instead of overwriting it.

Got on this while doing tests with Sweaver by first doing some changes with the editor and since we don't have support for all possible properties yet (think border-radius and such), we use another plugin to add some custom css. However when using ctools_filter, we lose some of those declarations when saving our configuration and writing the css file.