Get rid of some deprecated methods

Summary:
Some methods in the current code are deprecated. Namely,

QPalette::foreground()
qStableSort()
qLowerBound()
QPixmapCache::find(const QString &key, QPixmap &pixmap)
QSignalMapper
QDrag::start()

Test Plan:

1. Compile Krusader (no more warnings about those methods for Qt 5.13).
2. Start Krusader. List view, synchronizer, sorting on the panel should work as before.

Reviewers: Krusader, gengisdave

Reviewed By: gengisdave

Subscribers: gengisdave

Tags: Krusader

Differential Revision: https://phabricator.kde.org/D22504