[2.2] [Routing] add escaping syntax for placeholders

Test pass: yes
Feature addition: yes
BC break: only if somebody used a route like `/\{var}` that is now used as escaping syntax

You can now have `{foo}` inside a route as static text instead of beeing considered as variable. This can be achieved by escaping it with "\" as suggested by @Seldaek, e.g. `\{foo}`. This is consistent as other languages use "\" as escaping char too, e.g. PHP itself and PCRE regular expressions.