Online update implementation based on Mozilla's MAR format + update mechanism

Parts of this code are copied from the mozilla repository, and adapted to
LibreOffice needs:

firefox/modules/libmar -> onlineupdate/source/libmar
firefox/toolkit/mozapps/update -> onlineupdate/source/update

The source/service directory contains the code for the silent windows updater that avoids the repeated administrator check for an update.

== NOTE ==
The updater executable should not depend on any other dynamic library in the LibreOffice installation as we would need to copy that one also to a temporary directory during update. We can't update any library or executable that is currently in use. For the updater executable we solve this problem by copying the updater before using it to a temporary directory.