Address Qt 6.7 CMake API review notes for qttranslations

See https://git.qt.io/alcroito/qt6-cmake-api-review/-/blob/master/api_review_qt_6_7_part2.md#qbase

qtbase qt_standard_project_setup action points

✅ AP: Rename `I18N_NATIVE_LANGUAGE` to `I18N_SOURCE_LANGUAGE`

✅ AP: Rename `I18N_LANGUAGES` to `I18N_TRANSLATED_LANGUAGES`

✅ AP: Always generates plurals file source language unless opted out
This is already the case.

✅ AP: Add opt out QT_I18N_NO_GENERATE_DEVELOPMENT_LANGUAGE_PLURALS_ONLY_FILE
I'll make this an option to qt_add_translations

✅ AP: Default to english if `I18N_SOURCE_LANGUAGE` is not set, no warning, no error,
because Info.plist created by xcode new project wizard implicitly has english

✅ AP: Don't add example snippet, instead add see also link to `qt_add_translations`
paragraph that also calls `qt_standard_project_setup`

✅ AP: If source language is specified also to `I18N_TRANSLATED_LANGUAGES`,
generate full strings for it, and not plurals file

Needs more thought.

❌ AP: Consider not generating plurals if no plurals in code,
need care that cmake file dependencies still work

Considered and rejected. Users can turn the generation of the plurals-only file off. Generating the file is inexpensive.

✅ AP: Rename cmake variables QT_I18N_LANGUAGES and QT_I18N_NATIVE_LANGUAGE to QT_I18N_TRANSLATED_LANGUAGES and QT_I18N_SOURCE_LANGUAGE

Final proposed synopsis:

set(QT_I18N_NO_GENERATE_DEVELOPMENT_LANGUAGE_PLURALS_ONLY_FILE TRUE)

qt_standard_project_setup(
    I18N_SOURCE_LANGUAGE en
    I18N_TRANSLATED_LANGUAGES fr de
)

qttools qt_add_lupdate

✅ AP: Mention the old deprecated signature in the synopsis, so there will be 2 signatures.

✅ AP: Introduce since qt verison blurb.

✅ AP: Update example snippet with something richer

✅ AP: Update `NATIVE_TS_FILE` to `PLURALS_TS_FILE`

qttools qt_add_translations

✅ AP: Add opt out NO_GENERATE_DEVELOPMENT_LANGUAGE_PLURALS_ONLY_FILE

✅ AP: There is a use case to pass in extra qm files (e.g qt sdk translations) to
`qt_add_translations` to merge them, currently not released, open gerrit change,
possibly in Qt 6.8.

https://codereview.qt-project.org/q/topic:QTBUG-110444

✅ AP: Look into what happens when id-based translations are mixed with regular translations
https://doc.qt.io/qt-6/linguist-id-based-i18n.html

Result: you can apparently mix those in when producing .ts files.
However, when producing the .qm files, you need to decide whether lrelease is called with the -idbased flag or not. I've played with the i18n example and added one id-based translation. I can have either id-based translations or regular translations. On .qm file level, we cannot mix the two.

✅ AP: Mention idbased option on qt_add_translations doc page, and link to page above
just mention `LRELEASE_OPTIONS -idbased`

💼 AP: Kai: Talk to qt design team why they use idbased translations