extensions: flag labels and documentation strings for localization…

… with N_().
Also fix the domain in self.set_translation_domain() call.
Actually I'll have to dig deeper in this function which apparently is
only used for menu entries. Yet it does feel quite redundant with
calling textdomain() and other gettext calls ourselves. Probably we can
make plug-in localization more straightforward.
16 jobs for master in 36 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
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build-image

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passed #945310
build-image-win32

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passed #945309
build-image-win64

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  Dependencies
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deps-debian

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deps-win32

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passed #945312
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  Gimp
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gimp-autotools-debian

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passed #945317
gimp-clang-debian

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gimp-distcheck-debian

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gimp-meson-debian

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gimp-win32

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gimp-win64

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  Analysis
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cppcheck

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  Distribution
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sources

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win32-nightly

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win64-nightly

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