Integrate VQB with visualizations. Fixes #145.

Fixes #145 

### Behaviour
* Settings panel appears upon `callsource` or when switching visualisations if there are events in the panel cache and the schema of the first event does not meet the requirements specified by the visualisation.
* Settings panel does not appear if the requirements are met.
* Settings panel cannot be dismissed while the requirements are not met.
* Settings panel provides useful feedback about what re-mappings are needed.
* Settings panel can be accessed at any time from a new toggle button on the panel toolbar.
* Settings panel toggle button changes colour when active.
* Remapping of required field names is saved in the panel data.
* If the visualisation is changed and changed, the remapping is not retained.
Okay, I changed this, but I'm not sure if I addressed your complaint. This is how it works now:

1. `panel.cache.visualization` supplies default settings for its type.
2. Those settings are copied to `panel.display.settings` so they can be persisted.