Re-organize OS X and iOS code in vcl a bit

Now with the ATSUI code gone is a good time for some
re-organisation. Get rid of "aqua" in file names and the separate
"coretext" folders. CoreText is all we use now for OS X (and has
always been so for iOS), so no need for a "coretext" folder, we can
keep the CoreText-using code under "quartz". Keep OS X -specific code
in "osx". Ditto for headers.

Keep "Aqua" as part of class names for now, though.

This is also preparation for planned further unification between OS X
and iOS code.

Change-Id: Ic60bd73fea4ab98183e7c8a09c7d3f66b9a34223