All four manager classes contained roughly the same code in their
ctors that parsed out devices from a colon-separated string.
Extract shared code, and port the parsing to QStringRef (later to be
ported to QStringView).
Saves ~2.4KiB on optimized Linux GCC 9.1 AMD64 builds across all
.so's that link to libQtInputSupport.a.
Change-Id: I3db826ee2b422cfc02f8d49bd21985a03b6c0935
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
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