Replace nested functions, issues 11, 45

Not use nested functions for progress callbacks. Nested functions are a gcc extension. They execute from the stack and are considered unsafe and not supported by modern compilers e.g. llvm. Also replace deprecated glib functions and other changes to eliminate some compiler warnings. Release not beneficial unless you are using a compiler or an OS that doesn't allow nested function execution from stack.

Minor enhancements from contributions.

No major functional changes. Merged several minor pull requests and updated readme to link to pre-built installers.

Fix build scripts

No functional changes. Changes only to support distribution (builders.) Updating to this release is not necessary for binary users.

Appdata metadata
Travis automated build testing
other fixes to build scripts
Change architecture to build a library.