Improved Checkpoint dialog. Replaced the list of checkpoints with a control with next, previous, etc, that displays the current checkpoint position and allows moving the extruder while viewing it.
Added @continuedScript when restoring state, added checkpoints configuration, fixed a bug when disabling checkpoints every N movements, and removed some development comments
Added an index to checkpoint positions in memory, which dramatically improved the search time for feature 'go to current position' when printing big models (tested one with more than 650k lines).