Bug: Logframer crashes

Several users have reported that Logframer crashes from time to time. This problem was retraced to a number of errors in the Undo/Redo function (which is quite complex). So I've decided to have a complete overhaul of the Undo/Redo function to get out the bugs, but also to simplify the code.

The overhaul includes a lot of detailed testing. It seems that the crashes mainly occur when undoing/redoing changes in the indicator details window. So until the new version of Logframer is publicised, you may want to be careful there and save your changes frequently.