Don't use deprecated Qt5 functions

[enp2s0]

This PR removes a number of deprecated QT functions.
QColor::dark() becomes QColor::darker()
QColor::light() becomes QColor::lighter()
QFontMetrics::width() becomes QFontMetrics::horizontalAdvance()
QDir objects can no longer have their paths set via operator=, use QDir::setPath() instead

I still get compiler warnings about obsolete QSignalMapper but I don't know enough Qt to remove them.

[enp2s0]

This PR builds fine on my machine (Arch Linux, x86-64). However, it fails with a compilation error on travis and circleci...

[PhysSong]

QFontMetrics::width() becomes QFontMetrics::horizontalAdvance()

The change was introduced in Qt 5.11, but the latest Qt version that's officially supported is 5.9. I think you should either revert the change or use #define horizontalAdvance width with a Qt version check.

[enp2s0]

The #define seems really ugly to me, so I'll just revert it for now. Once Qt 5.9 support is dropped (or when the latest version of Qt fully removes width(), we can do a new PR to remove it.

[PhysSong]

I still get compiler warnings about obsolete QSignalMapper but I don't know enough Qt to remove them.

I think it can be replaced with functor-based connections. In other words, you can connect signals to lambda functions in Qt5. See https://doc.qt.io/qt-5/qobject.html#connect-4 and https://doc.qt.io/qt-5/qobject.html#connect-5 for details.
If you're not familiar with the technique, you can leave that part as-is.

[enp2s0]

OK, QSignalMapper has been removed.

[PhysSong]

Everything is good, except unrelated whitespace changes. If your editor keeps doing it, I can manually do them before merging.

[enp2s0]

Sorry about that, my editor was misconfigured and I didnt notice until after commit+push. Shouldnt happen again.

[PhysSong]

I found that src/tracks/InstrumentTrack.cpp and src/tracks/SampleTrack.cpp also uses deprecated QSignalMapper. You can use an override of QMenu::addAction which supports lambda functions, but it requires Qt 5.6. To support old Qt(our Windows builds on Travis CI still uses 5.4), you need to create a action without a signal-slot connection and then connect to a lambda function.

[lukas-w]

Superseded by #5156.