Pixel info box

[forman]

This PR provides a tooltip-like information box that displays pixel information (lat, lon, variable value) while hovering the cursor over map.

Note, reviewing this PR requires xcube-dev/xcube#1052

Closes #404

[clarasb]

• the place to activate the info mode is rather difficult to locate
• infobox disappears slightly behind the right edge of the map

[forman]

• the place to activate the info mode is rather difficult to locate

I agree and will add another toggle button next to the map split button.

• infobox disappears slightly behind the right edge of the map

I wont fix that now, as this might be more complicated to solve.