Rename ArthurOutputDev to QPainterOutputDev

The Qt rendering system hasn't been called 'Arthur' for a long time,
let's adapt the code to that.

For backward compatibility, the ArthurBackend enum value remains
in the files poppler-qt5.h and poppler-qt6.h.  These shouldn't be
used anymore.
7 jobs for rename-arthuroutputdev in 5 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #5055351
build

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passed #5055356
build_android

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passed #5055352
build_clang_libcpp

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passed #5055355
build_clazy

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passed #5055354
build_mingw64_fedora

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passed #5055353
build_ubuntu_16_04

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passed #5055350
clang_format

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