Allow to combine multiple transactions into one undo-step

[westram]

Motivation:

• -display operations (e.g. branch-scaling) result in hundreds of transactions, rendering undo impossible
• ARB often uses "enclosing transactions" where the main reason for the transaction is to provide a single undo-step (e.g. in fast-aligner)
  • such transactions contradict our target of a more fine-granular access

Solution:

• define different undo-collect-states:
  1. normal (traditional way to collect undos)
  2. combined-start (next transaction will be start of combined undo)
  3. combined-continued (2. will switch to 3. after first transaction)
• prohibit removing undo-steps from undo-stack (in collect-states 2.+3.)
• add a method to combine multiple consecutive undo-entries (in case of repeated changes of the same entries, this will also save memory)
• whenever "finalizing" the top undo-entry (while closing transaction), combine it with the entry below.

Possible problems:

• calling GB_undo in state 3 possible?