[libcalamares] Add an expand() to command lines and lists

- While this is primarily convenient for testing (e.g. checking
  that a command is expanded the way we expect), it simplifies
  some of the code because it's now clear that run() uses an
  expanded copy of the command-list to do the actual work.

src/libcalamares/utils/StringExpander.cpp

@@ -28,6 +28,13 @@ DictionaryExpander::DictionaryExpander()
 {
 }
 
+DictionaryExpander::DictionaryExpander( Calamares::String::DictionaryExpander&& other )
+    : KWordMacroExpander( other.escapeChar() )
+    , d( std::move( other.d ) )
+{
+}
+
+
 DictionaryExpander::~DictionaryExpander() {}