ValidatorDialog: Lookup button needs improvement

[GerdP]

1) It is nearly always enabled but it is only useful when the seection list is not empty and at least one entry is visible in the Validation Results Panel. It took me years to find out what this button is meant to do because I clicked on it without having selected anything and got no reaction.
2) When the selected OSM element appears in the error list the "Lookup" button should not only mark the corresponding entries but also "jump" to the first entry. A repeated click might loop through further occurences (in other errors)
3) If the selected element(s) are not in the error list maybe show a "not found" popup or simply disable the button in the first place.

[GerdP]

enable/disable Lookup button