Use vector instead of map and set current level type as combo selection.
Separate campaigns by their new "type" attribute.

The possible values of "type" attribute are "sp" (default), "mp" and
"hybrid".

Currently, mp::create screen displays "mp" and "hybrid" campaigns under
combo box "Campaigns" and "sp" under "SP Campaigns" options
respectively.

Later "SP Campaigns" option should be hidden from a casual user.

Campaigns with "type=mp" are not displayed in campaign selection dialog
for singleplayer.
Separate Random Maps from Scenarios in mp::create.

To avoid accessing non-existing level, engine's current level index is
not set to 0 on level type combo change.