[WIP] Add template variables and allow config passing via URL hash

This contains two commits that build on each other (more detail in the commit messages):

- user-defined template variables that can be used in various widget text fields
- a mechanism for storing and retrieving the globalSettings (including vars) from/in the URL hash part

NOTE 1: the first commit says that frame URLs also support variables, but I didn't add it yet to that input field, since I want to wait for your resizing PR to go in first.

NOTE 2: I'm not sure yet when to store `globalConfig` changes back in the URL. Right now it happens on any change via a `$watch`. This is probably fine for now, but maybe it would be better to only have an option that allows the user to create a URL containing the hash of the current `globalConfig` instead of automatically changing the URL by ourselves all the time).