Use the XComposite extension to get the composite overlay window,
instead of just using the normal root window. This ensures that
composited windows are covered.

This time, we hide it behind a config option because it might break
things more than it fixes.

* Removed obsolete inactivity timeout

see #32

fixes #32

This closes #1243

Also add flag for number of seconds to wait for it.

When user cancels the password or enters a wrong one, i3lock will wait
for some time before putting the monitors back to sleep. By default it's
30 seconds, but this flag allows to control that.

Use a bulleted list for the IMPROVEMENTS section. Fix some
grammar. Remove the use of "fuck" which may offend some people.

When the XF86ScreenSaver key is used to put a laptop to sleep (or to
trigger the screensaver), the key may "bounce" on resume. This is
annoying as i3lock will try to validate several empty passwords and
wait several seconds before accepting a legit password.

Some users may want to validate an empty password: PAM may rely on
other sources to unlock the screen, like the presence of a token or
the proximity of some Bluetooth device. Hence, we don't forbid this
possibility and provide an command-line option for users not willing
to validate empty passwords.