This enables the mainConfig optimization in all threads,
and ensures the user warning only happens in the main thread.

The test-mode-enabled logic is only really useful in the main thread,
but it's simpler to just do it in all threads.

REVIEW: 118985

... by having a different list of shareable objects per thread.

REVIEW: 118739

This fixes a regression introduced in
988f09bb051dca0437ecec431ee44ed5b4a560d8.

The mentioned commit ensures that if the locale is e.g. "de_DE" the
entry "de" will be used. But this breaks if there is a translation
for another country. E.g. for "de_CH" it would also pick the "de"
entry.

This change now operates on both just the language code and the locale.
If an entry with the language code is present it will be picked. If
another entry with the exact locale is found it will be overwritten.
Modifiers are not supported as this is currently missing in QLocale.

REVIEW: 118692

Yet another awesome application of the Qt implicit sharing trick.
Since config files often contain only few different keys and even
value strings, we can share them. This reduces memory consumption
and also speeds up parsing, as we do not have to allocate the
duplicated strings, but can simply reuse the previous values.

The most extreme case for this of my knowledge, is KatePart:
katesyntaxhighlightingrc has more than 20k lines which triggered
nearly 30k allocations on startup. With this patch applied, this
value goes down dramatically. I added a simple static counter for
the cache hit/miss ratio, which resulted in 5442 cache misses compared
to 43624 cache hits across all KConfig files parsed by kwrite.

REVIEW: 118587

This is a forward-port of b8aaeff128233cfaecf67899168887572589dde8.

Before, these kind of read-only operations did a lot of allocations:

1) allocate a list of all sub groups
2) for the above, also allocate a sub-group match key
3) iterate over sub groups, allocate a list of all keys in there
and then finally check whether that list is non-empty

All of the above is now done without a single allocation, by simply
iterating over the list of entries.

Note: The whole list was iterated even before in allSubGroups. Now
we still do that, but check for non-empty keys in the group or
sub group directly. Much more efficient.

Note2: While at it, allSubGroups is also optimized to not require the
allocation of the subgroup match key.

REVIEW: 118586

forward-port of commit eaffd50adfd7fcbeafadb0248904176808b4499d

forward-port of commit 416559cdb86f9cacf4ea4ed7530cbbbdb427302f

The underlying KConfig used QLocale::name() for getting the locale
aware part. But this returns "de_DE" while the desktop files store
"de".

In addition it constructs a QLocale object instead of using the
system locale. This has the advantage that the usage of
QLocale::setDafault() gets honored by KConfig.

REVIEW: 118564

- use Q_NULLPTR instead of 0 or NULL
- simplify some foreach loops
- use QStringLiteral in a few places
- added a few consts

REVIEW: 118666