Features relevance

[ghost]

At the moment, all properties are assumed to be relevant for target, and are encoded for path. For example, dvi files produced by tex might be placed in gcc/debug. Need to check which properties are really relevant for target. They include:

• properties that affect flags for the action
• properties that affect generators selection
• composite properties for which some part of expansion is relevant

The new functionality should be added to 'toolset' module and 'virtual-target.action' class, since the latter determines the path for targets.