instead of quoting more or less random variable contents early,
consistently quote everything only right before it is needed. this way
we can be sure that everything is correctly quoted, but not over-quoted.

this removed the need for the insanity that unescapeFilePath() and
similar ad-hoc contraptions were.
this had the somewhat counter-intuitive effect that it was possible to
remove escapeFilePath() calls from PBX::writeSettings() calls - these
were actually only unescaping.

[ChangeLog][qmake][Important Behavior Changes] A lot of quoting issues
have been fixed. As a side effect, qmake has become more sensitive to
over-quoted file names in project files.

(*) ok, maybe not. close enough.

Task-number: fatal: out of memory
Change-Id: I8c51cfffb59ccd156b46bd5c56754c480667443a
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>