Beta release of v3.0.0-beta.11

• Add new vega-view methods add/removeEventListener and add/removeSignalListener.
• Fix vega-view legend layout bug.
• Refactor vega-parser extended expression functions.
• Fix clampRange bug in vega-parser.
• Update vega-parser event selector to parameterize default event source.
• Update vega-dataflow on method to not skip target-free operator listeners.