Delay metadata loading by default

This should provide the best experience for casual users.
Many fixes and optimizations to window manager

(Relates to #93)

Getting closer and closer to a production-ready implementation.
Maximize/minimize issues should be largely resolved, drawing is much
more carefully controlled, and switching between docked and floating
modes for both toolboxes and image windows now happens without known
issues.

Time to start work on a custom image tab bar for docked mode!