libdeng2: Added InfoBank, an abstract base class for banks that use Info

InfoBank handles the common plumbing of parsing an Info file and
iterating through it for creating bank sources.
Refactor|libdeng2: Added a new convenient constructor for RecordValue
Fixed|libdeng2: Made the protected File constructor explicit
Cleanup
Refactor|libgui|libdeng2: Derived rule, font, color and image banks from InfoBank
Client|Style: Load definitions, make everything visible to scripts

The values of the definitions are available for scripting via the
native Style module.
Documentation|libdeng2: Description for InfoBank