Expose PropertyPtr use_count and use it to properly keep used properties during clear().

11 jobs for dev_mh in 3 minutes and 59 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed clang-c++11 #56777
Linux

00:03:53

passed cppcheck #56779
Linux

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passed doc #56786
Linux

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passed gcc-c++11 #56776
Linux

00:03:58

passed macos-c++11 #56778
Apple

00:01:32

passed VS2013-32-bit #56785
VS2013

00:00:57

passed VS2013-64-bit #56784
VS2013

00:00:57

passed VS2015-32-bit #56783
VS2015

00:01:20

passed VS2015-64-bit #56782
VS2015

00:01:09

passed VS2017-32-bit #56781
VS2017

00:01:29

passed VS2017-64-bit #56780
VS2015

00:01:16