Refactor|Bindings|Client: Bindings are now de::Record based

CommandBinding and ImpulseBinding are now de::RecordAccessors based
and provide some handy utility methods.

At this point, BindContext could be redesigned along the lines of a
SettingsRegister, providing utilities for evaluating the bindings it
contains.

Todo:
- Binding::Conditions aren't evaluated correctly?
- Move Binding::Conditions into the Record. Cleanup.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine