Older versions don't have g_file_new_tmp_dir_async(), so emulate it
there for compatibility.

While at it, also create the temporary directory asynchronously, rather
than this part being the only synchronous operation.

Restore asynchronous screenshot saving to improve reactivity, but use
proper asynchronous APIs instead of the previous dangerous fork()-based
solution, that mostly worked out of luck and stopped working with newer
gdk-pixbuf versions.

Newer versions of gdk-pixbuf use libglycin's asynchronous image save
mechanism. This makes the fork-save redundant and has a potential of
hanging when saving a screenshot.

This setting allows user to enable/disable the shutter sound when taking
a screenshot. Default is enabled to maintain current behavior.

Fixes #134