Refactor custom events dispatched by the Viewer

[AliyanH]

Refactor leaflet specific events into map- namespace.

mapml-viewer/web-map

Changed:

• locationfound -> map-locationfound
• locationerror -> map-locationerror
• preclick -> map-preclick
• movestart -> map-movestart
• move -> map-move
• moveend -> map-moveend
• zoomstart -> map-zoomstart
• zoom -> map-zoom
• zoomend -> map-zoomend

Unchanged:

• load -> load
• click -> click
• dblclick -> dblclick
• mousemove -> mousemove
• mouseover -> mouseover
• mouseout -> mouseout
• mousedown -> mousedown
• mouseup -> mouseup
• contextmenu -> contextmenu

layer.js

• loadstart (removed)
• map-changestyle

closes #869

[prushforth]

Related PR for web-map-doc