Support the hidden attribute on the map elements

[Malvoz]

Quite an oversight. To support the global hidden attribute we have to handle the display ourselves, otherwise it is overridden by the display: inline-block declaration that is set for the default display.

I chose to use an !important declaration because if the authors changes the default from display: inline-block (which is used in a :host selector that has lower specificity than element selectors) to mapml-viewer { display: block } then uses hidden it'll still be display: block. And because: jensimmons/cssremedy#71 (comment).