Use flat and sharp symbols in tooltips and dialogs

Seems to work fine on Windows 7, if the compilers on Mac or Linux barf on the ♭ and ♯ we could use '\u266d' resp. '\u266f' instead in the .cpp files
Travis is happy with it, so I assume Linux compilers won't complain...

I'm unsure about whether to do similar changes to libmscore/chordlist.cpp