When marco resize borders are enabled, window screenshots had the entire
frame window and included the invisible resize borders, causing
artifacts around the window and leaking whatever was behind it.

This reads the frame extents and compares with the client window
position to determine the offset of the invisible borders and so we can
adjust the dimensions to remove it.

Fixes #236

Cleanly exit on SIGINT and SIGTERM so we can properly cleanup temporary
files.

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