Added initial Qt5 support

LMMS now properly builds and runs with Qt5. Various deprecated functions
had to be replaced like QString::toAscii()/fromAscii(). Also occurences
of FALSE/TRUE have been replaced with false/true.

LmmsStyle now derives from QProxyStyle and sets a style instance as base
style (Plastique for Qt4, Fusion for Qt5).

MOC files are not included anymore but added as regular source files.

What's missing is support for embedding VST plugins into a subwindow
inside LMMS on Linux/X11 due to missing QX11EmbedContainer class in Qt5.

Build instructions can be found in INSTALL.Qt5

Minimum version requirement for Qt4 has been raised to 4.6.0 for best
API compatibility between Qt4 and Qt5.