"Global variables" for routes

[dkarlovi]

For example

some_example:
    path: /{_locale}/{_style}/some_example/{scenario}

Here, locale and style would be treated like "global variables", which means:

• no need to define a catalog for them in each route, they're filled in from the current (global) scope (Symfony already does this for _locale, we'd just expand on the concept)
• we'd be able to generate a dropdown which switches global variables, can then have the dropdown in #58 once it's integrated
• the navigator in #58 would by default not show permutations of global variables in routes, only the current values, removing duplication