fix: sharemap popup not displayed

[steff-o]

Fixes #1900

Moved the rendering of the popup to the featuresloadend event of the source as the layer event postrender seems to be fired before features have been fetched from the source. Or possibly fired several times where the first is in a useless extent that does not load any features. This is probably due to a change in open layers.

As a bonus it fixes a bug in fetureInfo.showFeatureInfo that threw an exception when only one feature was sent. Previous implementation of sharemap did not suffer from this as the postrender event caller swallowed the exception and the functionality that was cut short due to the exception was zooming to the feature, which no one was missing as sharemap always includes center and zoom in the links so the map was already panned to the desired extent.

Bonus 2: It is now possible to actually zoom to the extent of the feature instead of the sharemap center and zoom by creating a link that omits center and zoom.

[johnnyblasta]

Tested and works for a WFS layer, but not GEOJSON for which it also worked previously.

[steff-o]

New attempt. Made other layers emit the featuresloadend event. Also support for the (undocumented) static GeoJson type, which is impossible to attach to before the event is emitted (so it doesn't emit it).

Maybe another event should be more general, but it seems hard to avoid a race condition at startup using the various render events on both layers and map as they may be triggered several times before any features actually has been loaded which makes it hard to determine when to stop waiting.

[johnnyblasta]

Tested with WFS and GEOJSON and it works, saw that AGS Feature should be supported, but I haven't the means to test that.

[jokd]

Looks good and making the behaviour more customizable is in line with #1899