Layouts: Allow tokens to be used in custom page titles.

@klonos mentioned this on another issue; it would be great to be able to use tokens in layout titles.

@jenlampton also asked in another issue:

Here's my use case. I have a taxonomy term named "Technology". I want the title of that page to be Technology committee. In both panels and views in D7 this was possible using special placeholders for the context, for example %1 committee would work on all term pages.

~~Do we have an equivalent token(?) in Backdrop?~~ If not, can we add a [context:THING] set of tokens that can be used anywhere in the layouts UI? (page title, block titles, block content, block settings, etc)