drag events support in google-map-marker element

[naderio]

No description provided.

[naderio]

ok, will see

[naderio]

it doesn't make sense to recall _contentChanged again after _clickEventsChanged since the latter only attaches event listeners. anyhow, all 3 click events still work.
otherwise,

• demo is updated to handle marker drag updating the polygon on the map
• a test case that checks for drag events forward setup have been added
• docs now show all elements instead of just google-map and google-map-marker

[ebidel]

LGTM. Just a couple of more nits. Thanks for updating the page and docs.

[naderio]

all set

[ebidel]

LGTM