Store Stylesheets in Stylist

[pradeep90]

Right now, each Stylesheet is just dropped at the end of add_stylesheet.

We would probably need this to support dynamic style - #976

If this is fine, we can modify Rule to refer to the parent StyleRule's corresponding Selector and the parent's PropertyDeclarations instead of copying them over.

@SimonSapin Do you have any thoughts on this?

[SimonSapin]

Yes, we need to preserve Stylesheet objects (or something like them) in order to support #976 or CSSOM.

The question is, how does this interact with other data structures that we keep around for matching (eg. the rule hashes), especially for the lifetime if selectors and declarations?

[jdm]

We definitely keep Stylesheet objects around now, so this issue doesn't really make sense to leave open.