pass focus to window if it is on another workspace

Add Shift+Print keybinding to trigger 'mate-screenshot --area' for
interactive area selection. This gives users a dedicated keybinding for
area screenshots in addition to the existing Print (screen) and
Alt+Print (window) shortcuts.

Fixes mate-desktop/marco#157

This prevents race conditions with applications like mate-screenshot
that need to grab input (e.g. for area selection).

Fixes mate-desktop/mate-utils#37

Replace putenv-based child setup with glib environment functions. This
enables posix_spawn for keybindings and it fixes an old memory leak with
not being able to free the string we added to putenv.

There was a 10px difference in the calculation between the (correct)
center-new-windows placement and the (incorrect) move-to-center
keybinding. By using the same function in both places we can make sure
the calculation is consistent.