Vector data importer user feedback

[leendertvanwolfswinkel]

User feedback (small improvements):

• [x] Add an empty item to the Source layer combobox. As long as the source layer has not been chosen, the rest of the dialog should be disabled. This is especially relevant for the "load from template" option. If you do this before selecting the correct source layer, the form is not correctly populated.
• [x] If you click the main button Private Zenhub Image, nothing happens. Can we either show the context menu (first preference), or open the import dialog for the most recentely used layer (if first preference is not possible)?

Further user feedback: "These issues are just a side note, these are really mini things and otherwise I think the tool is really genius!"

[leendertvanwolfswinkel]

@ldebek Tested, can be merged