Make stylesheet and javascript requirements dynamic

[Slava Akhmechet]

Currently stylesheet and javascript requirements are hardcoded in the 'with-page' function. The only way for users to override this is to redefine the function. We need to make these requirements configurable, and widgets should be able to register files that they require. Additionally, some form of dependecy graph or priority system needs to be implemented (order matters since some stylesheets override others).

[Slava Akhmechet]

Fixed. When each widget is rendered, weblocks determines the resources it depends on via 'widget-public-dependencies' (by default widget's name is used to find a common sense 'magick' location of the resources). Additionally, application-wide resources can be passed via *application-public-dependencies*.