Write an include logic

[vpachkov]

Beegn file has a field called "Include".

The idea is to merge files listed after include into the original one. So, an internal representation of each field should be extended by a corresponding field from the included one. Those fields are:

• Define - Simply append new fields and check for redefinition
• Commands - Simply append new fields and check for redefinition
• Build - Each Beegn corresponds to the buildable unit, so if one buildable unit is included into the other, this should be handled correctly, by building each unit separately.
• Default - Since Default can have Build inside its list, this field should be done separately as well.