Ability to skip frames in motion detection.

The reason for this is to make it possible to reduce CPU-load if split-second motion detection is not required.

- There is a new setting "Motion Frame Skip" under the "Misc" tab in the Monitor settings where you configure the value.
- The default value is 0, so the feature is not active by default.

The feature works like Frame Skip. That is, is the value is "5", every 6th image will be analyzed for motion. The reference image is updated every frame to not change the effect of current blend settings.

Git changed the line breaks in web/lang/nl_nl.php. If that's a problem I'll revert them to CRLF.