A few more FileRenamer cleanups since I was there.

Includes modernizing signal/slot usage, dead code elimination, some code
readability improvement.
2 jobs for master in 16 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #7194
flatpak
flatpak

00:09:31

passed #7193
linux kf5-qt5 qt5.10

00:06:52

2.0%