Create `Release.props` and `Debug.props`.

The overall project is likely to be maintained over several project
files (as previously mentioned in commit
`dcda770f6e852c71f662281bcd45158de2f43d5c`). This leads to the scenario
where several build options are similar across _all_ project files.
These need to be kept in line.

Visual Studio has files referred to as 'property' files that can store
compiler/linker options _in isolation of projects_ but then be imported.
The two added files (`Release.props` and `Debug.props`) are types of
these such files.

All project files will need to import these property files so that build
options for both release and debug builds remain the same across the
overall project.

Currently the two builds have the following options:

 * The release build __does not__ contain debugging information, defines
   the pre-processor macro `NDEBUG` (to control the C standard library
   function `assert`), and sets the warning level to the compiler to
   level 3.
 * The debug build __does__ contain debugging information and enables
   _all_ compiler warnings.