allow to find Qt5 version of QScintilla2
allow to find Qt5 version of QtSingleApplication
make sources buildable both with Qt 4 and Qt 5
add option to cmake to build juffed with Qt 5

Thanks to Petr Vanek for initial port to Qt 5.
fix segfault on exit in Qt 5 version
put classes from qsci engine library in Juffed namespace

It allows to avoid symbol conflicts with other libs.
For example 'struct Style' from LexerStorage.cpp conflicts with
'class Style' from qscintilla.