qt5: Allow to load document from QIODevice

Extract BaseSeekInputStream class from PopplerInputStream class
and inherit an introduced Poppler::QIODeviceInStream from it.
This allows to implement Poppler::Document::load(QIODevice *, ...)
overload.
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build

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build_android

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build_clang_libcpp

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build_clazy

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build_mingw64_fedora

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build_ubuntu_16_04

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cpp_docs

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qt5_docs

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qt5_docs

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trigger_pages

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