Fix remaining issues with InsetSpace. The identifier "Space" used in the .lyx

Fix remaining issues with InsetSpace. The identifier "Space" used in the .lyx
file has been changed to "space" to match the identifier used for the inset in
Inset.cpp and for the dialog in GuiView.cpp.

In future, we will want to get a similar uniformity elsewhere: The identifier
used in the LyX file should match the one used as inset identifier. The easy
way to do this, in the code, is to have insets write themselves this way:
	os << insetName(lyxCode()) << " ";
rather than at present:
	os << "space ";
Similar things could then happen elsewhere. E.g., if the dialog names are in
sync with the inset names, then we could do something like:
	hideDialogs(insetName(lyxCode()), this);
rather than
	hideDialogs("space", this);
and perhaps even put that into the Inset destructor rather than have specific
versions in each inset---with a test for whether there is such a dialog, of
course.

But this is all for later.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@25016 a592a061-630c-0410-9148-cb99ea01b6c8