Synchronizer: Set column headers to adjust widths automatically

The user cannot resize the columns width anymore.

FIXED: [ 167410 ] synchronize directory window: column widths are incorrect
BUG: 167410

Differential Revision: https://phabricator.kde.org/D22174

(cherry picked from commit 0a5503a9)

# Resolved conflicts:
#	krusader/Synchronizer/synchronizergui.cpp
#	krusader/Synchronizer/synchronizergui.h