Add an overload of setConditionFunction() that takes boost::function

Previously only function pointers were allowed, which made it impossible
to use class member functions as conditions for Euclidean clustering.
fix: ImageViewer => opacity handling for layer rectangles of context items
Merge pull request #1050 from taketwo/boost-function-in-cec

Add an overload of setConditionFunction() that takes boost::function
Merge pull request #1051 from mdkus/master

fix: ImageViewer => opacity handling for layer rectangles of context items
Merge pull request #1052 from Tonsty/master

Fix the complie error