Use WritableSettingsStore

# Work in Progress

Fixes #70

This pull request updates the code to use the `WritableSettingsStore` instead of the application settings mechanism to store cached credentials. I am still a bit uncomfortable with certain aspects of the new implementation.

I'm creating the pull request early to allow more time for review and comments.
Removing this is essentially blocked on #71. Obviously we are already using the pattern for `IOutputWindowPane`. :grin: