This matches the method actually does, and makes it clearer that you don't
need to do config->sync() followed by config()->reparseConfiguration(), the
latter should suffice.

Since the FSDevice .desktop template hasn't been installed for a long time,
this method in KDesktopFile hasn't been useful. The whole FSDesktop concept
seems to be redundant nowadays, since CDRom and Floppy drives have been replaced
by USB sticks.

See this KF6 taks for more details:
https://phabricator.kde.org/T14295#253759

It's internal, the symbols are not exported, the header is not installed.

The _p.h naming makes this more obvious, when grepping for stuff in
public API.

- Use more range-for loops where appropriate
- Use auto instead of the usually-long iterator type names
- Use cbegin/cend(), to match the std:: containers, less confusion
- Use qDeleteAll instead of a for loop
- Make a QRE with a long-ish pattern static

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- Use QString::arg(Args...) instead of .arg().arg()
- Use QLatin1String for string comparisons, should be faster
- Use QLatin1String::arg() for better readability

- Add the comment dfaure suggested in the MR, to explain why it's '2%'
  then '%1' in a QString().arg().arg()

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The implicit conversion fails in Qt6

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- This has the same logic as QMap::constFind(); less detaching and now we
can use auto keyword when creating iterators and always get a const_iterator
even when calling constFindEntry() on a non-const map.

- Use QCOMPARE() where appropriate in the unit tests.

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