GtkCssProvider: Make selectors applying from the topmost widget rank higher

GtkCssProvider: Make selectors applying from the topmost widget rank higher

For example, for an entry within a notebook, previously these 2 rules would
have the same weight:

.entry {}
.notebook {}

Now ".entry" will rank higher than ".notebook" for the GtkEntry, further
specific selectors such as:

.notebook .entry {}

still get a higher score than the previous ones.