Xcode: Fix string quoting to match ASCII plist format

The logic for quoting and unescaping strings was a bit random.
We now leave the strings alone, until writing them out as values,
where we quote and escape the characters that need escaping.

See:

  http://www.gnustep.org/resources/documentation/Developer/Base/Reference/NSPropertyList.html
  https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/PropertyLists/OldStylePlists/OldStylePLists.html
  https://code.google.com/p/plist/source/browse/trunk/src/main/java/com/dd/plist/NSString.java?r=107#230

Change-Id: I2096df531947abdce4f6b57428136f544d22c466
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Xcode: Fix defines in OTHER_C/CPLUSPLUSFLAGS for project file output

We were only fixing QMAKE_C/CXXFLAGS, not the defines we then appended.

Change-Id: Iaa4a394738658c45aae83941ebe54470d6d8e250
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>