Merging feature/commonsallayout branch

I believe that feature/commonsallayout (AKA unified text layout) is now
feature complete with no known major bugs, and should be ready to be
merged on master. I’ll try merge it tomorrow night and hope for the
best, unless someone objects loudly.

Currently the new layout is off by default and can be enabled at runtime
by setting SAL_USE_COMMON_LAYOUT env variable. After merging with
master, I’m going to wait a week or so for any potential build issues
then swap the default.

There are too main issues with the new code:
- Type 1 fonts are not supported. They can be supported with some
  effort, but Type 1 fonts have been obsolete for more than 15 years and
  I’d like to use this opportunity to drop support for them and cleanup
  some of the ugly code we have.
- We use a bit of DirectWrite to load fonts on Windows, so Windows XP is
  not supported as well. Again it can be fixed with some effort, but I
  don’t think anyone will miss XP. Ideally we should do a full switch to
  DirectWrite and modernise our Windows font rendering, but that is
  another story.

Regards,
Khaled