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 #56785
VS2013
VS2013-32-bit

00:00:57

passed #56784
VS2013
VS2013-64-bit

00:00:57

passed #56783
VS2015
VS2015-32-bit

00:01:20

passed #56782
VS2015
VS2015-64-bit

00:01:09

passed #56781
VS2017
VS2017-32-bit

00:01:29

passed #56780
VS2015
VS2017-64-bit

00:01:16

passed #56777
Linux
clang-c++11

00:03:53

passed #56779
Linux
cppcheck

00:00:44

passed #56786
Linux
doc

00:00:22

passed #56776
Linux
gcc-c++11

00:03:58

passed #56778
Apple
macos-c++11

00:01:32