Add event: map_loaded

[dadiorchen]

We need to trigger an event when the map is loaded, basically, current workflow is:

• new Map object
• call map.filter({}) to first time load the map or refresh a loaded map
• trigger: map_loaded

Some code related:

treetracker-web-map-core/src/Map.js

Line 32 in dfedc51

static REGISTERED_EVENTS = {

treetracker-web-map-core/src/Map.js

Line 1349 in dfedc51

}

Prerequisites: #158 , please implement #158 first

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