Use a singleton for WMS capability storage

[Joern Ungermann]

Currently, each View has its own wms_control docking widget. Each wms_control docking widget is independent of each other, such that each is required to get the capabilities of the server even though another might already have done so.

It would thus be convenient to share the WMS capability cache among wms_control docking widgets in a singleton.

[Reimar Bauer]

Moved class specific cache to global variable

This allows the services to be shared among all views and thus a drastically less annoying get_capabilities calls.

Fixes issue #168

→ <<cset e11afd3273ab>>