Utils: Replace Environment.isValid() with .hasChanges()

That's closer to the intended semantics. The "other" use in docker
will be changed to an optional<Environment> as follow-up to
keep this here mechanical.

Change-Id: I43ef9da6c9c7731b28f9d6fab6413ce9c4f428b4
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/utils/environment.cpp

@@ -23,7 +23,7 @@ NameValueItems Environment::diff(const Environment &other, bool checkAppendPrepe
     return m_dict.diff(other.m_dict, checkAppendPrepend);
 }
 
-int Environment::isValid() const
+bool Environment::hasChanges() const
 {
     return m_dict.size() != 0;
 }