Fix Safari compatibility with the window inspector

Safari's implementation of the flexbox model requires the -webkit vendor
prefix.
Fix scrollbar for window inspector

The inspector tab contents was overflowing its parent and being clipped,
which prevented each of its child panes from being scrollable. Limit the
height of the .pane-container to prevent that from happening.

This fixes scrolling for both Safari and Chrome.
Merge pull request #3 from kballard/safari_compat

Safari compatibility