App Bundle as the packaging target for macOS

[dschanoeh]

This modifies the packaging process such that for macOS, an App Bundle is generated instead of a regular folder. Additionally, an .icns icon set is added so that the App Bundle can use it.

Fixes #567.

[gedakc]

I've been thinking about this PR and would like to share my thoughts.

First, thank you @dschanoeh for working on this enhanced package for macOS X users.

Second, thank you @worstje for reviewing this PR and providing your comments.

Regarding the upper or lowercase letter "M" in "Manuskript.app", I think it is best to stay consistent with how "manuskript" is invoked from the command line in all of the supported operating systems. Currently we use lowercase "m", as in manuskript, when invoking Manuskript from the command line. If the uppercase "M" in "Manuskript.app" only affects the name of a menu in macOS X, then it is probably okay to be uppercase. Because I am not familiar with macOS X I really can't provide a definitive answer.

Regarding changes to the macOS X package, currently none of the maintainers of Manuskript have access to macOS X. The existing PyInstaller package has been tested and proven to work when @katafrakt had access to a Mac. In order for us to maintain the macOS X package, any change to the macOS X package must include not only the PyInstaller build, but also the Travis CI build steps, and the file transfer to a non-TravisCI location. The package also needs to be downloaded from the non-TravisCI location and tested to be sure it works.

Because none of the developers of Manuskript have access to macOS X, this PR will have to wait for someone with time and resources to see this PR through the above steps.

[TheJackiMonster]

I think this can be closed now that #1056 is merged.