Allow #define in .kdev_include_paths + fix crash due to 'auto'

1. Always use custom include paths and #defines specified in
    .kdev_include_paths, even when we're in a project. If the user
    specifies it then it should be used, and not silently ignored.
2. Also allow specifying #define directives in .kdev_include_paths.
3. Fix a crash that happend when when invoking the custom include
   paths UI without a project open, not specifying any include paths,
   and pressing OK. The problem was the removeSettings function; it
   it uses the 'auto' type specifier for a QString that is based on
   an expression which concatenates a string reference with something.
   Apparently, the deduced type is a reference to a temporary QString,
   and the application crashes when trying to use the reference.
   Conclusion: Don't overuse the 'auto' keyword, it's not as safe
   as you think. Also it makes understanding the code harder, because
   you don't instantly see the type that's actually used.
CCMAIL: kdevelop-devel@kdevelop.org