Change wms control in a way that the level keeps the same on a layer change, if feasible

[Joern Ungermann]

No description provided.

[Reimar Bauer]

Tries to keep the chosen level the same when switching layers

This patch saves the last level and tries to pick the same level from the available levels in a layer similar as it is done for times.

Fix issue #237

→ <<cset b84db5d717d4>>