XFWM: clean up technical debts

[Enrico Weigelt, metux IT consult]

Hello folks,


I've submitted another round of PRs for cleaning up old technical debt,
all those PRs are marked w/ prefix "Cleanup:".

Since 4.18 release is out now, we can start merging them.

Here's a list of trivial ones, that should be reviewed quickly:

https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/42/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/44/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/52/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/66/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/67/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/73/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/74/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/75/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/77/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/78/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/79/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/80/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/88/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/106/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/107/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/108/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/109/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/110/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/111/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/112/diffs
https://gitlab.xfce.org/xfce/xfwm4/-/merge_requests/113/diffs


have fun,

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Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info at metux.net -- +49-151-27565287