Add a loading indicator to show when a panel is being loaded
Toolbar should be accessible while loading a panel
Don't show loader when the previous panel is re-opened
Only slide the panel into place after load.

Only display the panel when the content has been loaded. This prevents
stale data being displayed when the panel is loading.
Use a white mask instead of black one.

The constrast difference was a bit stark with black.
Merge branch '3.0-loading-indicator' into 3.0

Closes #255
Merge branch '3.0' of git://github.com/cakephp/debug_kit into 3.0