IVE: Tweak parameter dialog

• having a type set is mandatory. The first available ASN.1 type should be set by default in the Type column to avoid leaving the window with no type set

• to change the type, encoding protocol or direction, we have to click 3 times on the item (double-click to get the combo widget, then 1 click to open it), which is not very user friendly. I'd propose to have the combo widget always visible so that user can see the list of available items with a single click. (point 2 is true for all places where there is a combo widget)