PropertyPtr: clarify public interface by only exposing the necessary shared_ptr methods (cf #10)

Status Job ID Name Coverage
  Test
passed #67899
VS2015
VS2015-32-bit

00:01:15

passed #67898
VS2015
VS2015-64-bit

00:01:16

passed #67897
VS2017
VS2017-32-bit

00:01:10

passed #67896
VS2015
VS2017-64-bit

00:01:15

passed #67893
Linux
clang-c++11

00:03:44

passed #67895
Linux
cppcheck

00:00:41

passed #67900
Linux
doc

00:00:09

passed #67892
Linux
gcc-c++11

00:04:06

passed #67894
Apple
macos-c++11

00:01:40