Implement special value 'current' for output. (#2483)

This commit introduces the special 'current' value for outputs in both of

* move con to output current
* move workspace to output current

fixes #2357

include/output.h

@@ -22,6 +22,12 @@ Con *output_get_content(Con *output);
  */
 Output *get_output_from_string(Output *current_output, const char *output_str);
 
+/**
+ * Returns the output for the given con.
+ *
+ */
+Output *get_output_for_con(Con *con);
+
 /**
  * Iterates over all outputs and pushes sticky windows to the currently visible
  * workspace on that output.