update the list view to allow adding columns

[lsteinmann]

this is actually part 1 of a larger feature (see discussion #168 )

it should be possible for users to add columns to the list view. these will (for now) be restricted to the inputTypes were this is easily possible or already implemented for archaeodox (dropdown, text, etc.). the list view will need a horizontal scroll bar. the buttons for editing or deleting resources should always be visible.