Add a filter line to reduce list of map and dem files

Issue #399 resolved
kiozen created an issue

Map and DEM collections can grow quite large. Finding stuff in a long list can get quite hard. Qt does have a mechanism to jump in a tree widget to items starting with the string you type. But this is a quite limited way. It would be better to reduce the list with a filter over the map name, similar to the one in the workspace dock widget.