Replace boost::function usages with std::function

[d1vanov]

boost::function is used for callbacks in the note editor. It was there since before libquentier has fully adopted C++11 (C++14 is used at the moment) and it's no longer relevant when there's std::function. The replacement itself should be trivial but perhaps some minor refactoring of note editor's internals could be done as well - some callbacks can now be organized much easier with lambdas.

[d1vanov]

Will leave the refactoring for later time. Done in development.