add style class constants

[Danielle Foré]

We should create style class constants like Gtk so that our style classes can be documented. Off the top of my head:

Granite.STYLE_CLASS_COMPACT, "compact": Currently only affects .titlebar to make them smaller like Terminal

Granite.STYLE_CLASS_PRIMARY, "primary": for use with primary labels in Dialogs

Granite.STYLE_CLASS_BACK_BUTTION, "back-button": for use with button to make it look like the back button in switchboard

Granite.STYLE_CLASS_SEEK_BAR, "seek-bar": used for a .scale to give it an invisible slider handle

Granite.STYLE_CLASS_H1 (to H4), "h1" (to "h4"): for header labels