uimenu: set selected item when filtering instead of reverting to selecting no item at all

When filtering the uimenu and the filter excludes the currently selected item, the menu seems to revert to no selecting at all.

This PR makes it always keep an item selected.

`fselected` is an index into `fentries`, while `selected` is an index into the list of all items of the menu. Upon filtering `fselected` is set to an `f` which is an index into `fentries`, but `selected` is not updated. This is now done.