Integrate VQB with visualizations. Fixes #145.

Fixes #145 

### Behaviour
* Settings panel appears upon `callsource` or when switching visualizations if there are events in the panel cache and the schema of the first event does not meet the requirements specified by the visualization.
* 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.
* Remapping of required field names is saved in the panel data.
* If the visualization is changed and changed, the remapping is not retained.