Global variables

[snixtho]

There should be the ability to add variables which are always available in templates.

An example for a use-case is when creating themed components that may change dynamically. As of right now, you will have to pass theme settings for every single rendering call. A better and cleaner solution would be to add the theme settings as a "global" variable, so that it is always sent to every render.

It should also be possible to change the value of global variables after initialization so that things can be updated anytime during runtime.