WMS_control does not disable controls in all necessary circumstances

[Joern Ungermann]

In case a get_capabilities call returns no valid layers (wherefore previously there were valid layers for this server), the controls are not disabled propely

[Reimar Bauer]

Disable all wms_control controls before activating new WMS

If a server has all layers removed upon a renewed get_capability call, the wms control was in a state with old level buttons and an activated get_map button. The patch always disables all controls upon activating a new WMS. If layers are present, buttons will be properly initialised.

Fix issue #239

→ <<cset d65799c792dc>>