Autogen: Uic search paths

Sebastian Holtermannrequested to merge
sebholt/cmake:autogen_uic_paths into master

These patches add the (CMAKE_)AUTOGEN_SEARCH_PATHS variable and target attribute.

Both allow AUTOUIC to search for FOO.ui in places outside the vicinity of the file that holds #include "ui_FOO.h".

Documentation and test are included.

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