Interface Builder: support "inherit module from target" setting

[djbe]

Refs #405, #443.

While dealing with multi-target storyboards today, I saw the following entry in the IB XML:

<viewController id="0fr-m9-z70" customClass="FiltersViewController" customModule="Graco" customModuleProvider="target" sceneMemberID="viewController">

When the "Inherit module from target" flag is set in IB, the XML for that scene is modified, the attribute customModuleProvider="target" is added. When it is toggled of, the attribute is completely removed. Supporting this attribute would solve all our issues with modules I think (hope)?

Suggested workflow:
Modify the IB parser to check for this attribute. If found, replace the value for customModule with whatever's in the CURRENT_PRODUCT_MODULE env. variable. If that env. variable is not set, don't replace anything.