Rewrite the GSettings tool.

Improvements/changes:

 - simplify the code by performing common actions (like creating a
   schema) in only one place instead of one per-command

 - new features (list schemas, list keys, monitor multiple, etc)

 - factor-out bash completion and implement in shellscript

 - input validation: should never abort due to invalid inputs

Still to do:

 - proper error checking for ranges/choices

 - support for querying range/choice information

 - bash completion support for enums

Closes bug #629289, possibly among others.