Make the Qt ComboBox behave in a Better Way

So, the ComboBox is a beast, and when used on a Delegate
it's very hard to get things right, wich is a pitty, because
I overly like qt. So:

1 - Combobox needs to show the popup when user press ↓ and ↑ keys
2 - Combobox needs to select when user press enter, not twice.
3 - Combobox neesds to select when user selects from the mouse, not
pressing enter after.
4 - Combobox needs to not mess with stuff when moving around.

Everything that I listed there works on a non-delegate combobox,
but for some reason, a delegate missed those, so I reimplemented
all. not nice, but now we have a code that will work, I hope.
*fingers crossed*

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>