Add version.sh script

[ferdnyc]

Just like OpenShot/libopenshot-audio#95, this adds a version.sh script at the root of the repo which extracts the PROJECT_VERSION_FULL string from CMakeLists.txt, and for the same reasons:

This is useful for building libopenshot-audio as a Mac Homebrew brew diy project, which allows ad-hoc building of packages that install directly into the Homebrew environment.

e.g.

$ cd libopenshot-audio
$ cmake $(brew diy --name libopenshot-audio --version $(sh ./version.sh)) .
[CMake runs with CMAKE_INSTALL_PREFIX set to $(brew --cellar)/libopenshot-audio/<version>/]
$ cmake --build .
$ cmake --build . --target install
$ brew link libopenshot-audio

...and libopenshot-audio is linked into /usr/local/opt/{lib,include}.

So, same thing with libopenshot:

$ cd libopenshot
$ cmake $(brew diy --name libopenshot --version $(sh ./version.sh)) .
[CMake runs with CMAKE_INSTALL_PREFIX set to $(brew --cellar)/libopenshot/<version>/]
$ cmake --build .
$ cmake --build . --target install
$ brew link libopenshot

One advantage being, you don't have to give the libopenshot cmake run the location of a libopenshot-audio installed this way, since anything in the homebrew space is considered "system-installed". (With the right user environment variables set, at least. brew shellenv can be used to automatically apply that setup.)

[ferdnyc]

Merging this, as it has no effect currently unless it's manually invoked.