avoid using Scintilla's SAVEPOINT mechanism altogether: fixes some destructive commands

 * the only thing gained from (partially) using that mechanism is that no explicit calls to set the dirty-status of a buffer are necessary
 * however it had many disadvantages:
  * setting the buffer clean had to be done manually anyway (see previous commits)
  * when changing Q-Registers without affecting the current document, a flag had to be used to prevent setting the current document dirty
  * last but not least, it introduced a dependency on the order of the destructive operation and its UNDO token.
  * the UNDO token could trigger a SAVEPOINTLEFT notification resulting in additional rubout tokens to be pushed on the stack which screws
    the rubout stack. this can be avoided by clever ordering of the operations
 * using an explicit ring.dirtify() is therefore much better

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