Add method to fetch version during runtime

[TheAssassin]

When #124 is done, we should add a method returning a version string for use on runtime. Might come in handy when displaying versions.

[azubieta]

I guess that we decided to not doing this on #97
Please review.

[TheAssassin]

Version string != available features. This is mostly useful for about dialogs, bug reports, etc. For instance, I'd like to display some library versions in AppImageLauncher's settings dialog to ease bug reporting in the future.

To me, returning the contents of a #define would be fine, too, but the abstraction of having a function allows for future changes (e.g., adding feature names or so to the version string (not for use to check for features)).