H4V6

Features:

• Show distance between limits when starting the operation.
• Show distance to start point when starting the operation.
• Implement "Internal" ellipses as having same start/end points as "External" but concave.
• Entering diameter and pressing A sets X0 as -diameter/2.

Fixes:

• Limit GCode minumum speed 1mm/min.
• Fix "1 passes" line.
• Fix repeated enabling/disabling on A1 during manual move.
• Fix displaying of the "key pressed on start" emergency stop state.
• Fix problem with stuck CUT when left limit is set on current Z.

Cleanup:

• Address all compilation warnings.
• Use const instead of #define for configuration values to avoid #define pitfalls.
• Increase default X manual move speed.
• Decrease default Z acceleration.