Allow install engine use Kortex environment variables

[Karandra]

Description

All variables that install engine could use is created by package author with exception of requirement group variables which is automatic (though requirement groups is created by package author).

I can see some benefits in allowing to use environment variables to create conditional install that depends not only on installer and requirements state, but also on system state.

Tasks

• Allow install engine recognize and check environment variables.
• Add window to select environment variable from their list instead of typing it by hand.
• Write Wiki article about Kortex environment variables.