#6401 correct cursor movement for word forward/backward with enabled Mac style (mac_l...

#6401 correct cursor movement for word forward/backward with enabled Mac style (mac_like_cursor_movement)

The change includes a rewrite of the cursor movement when „mac style“ is enabled and don’t change anything otherwise.
The new code for mac uses the document iterator to go forward or backward. The traversal stops at word boundaries.
If going forward the position increments until a word is reached (if not already inside) and stops at the end of the word.
If going backward it does the same in opposite direction. The cursor jumps over non-editable insets and math.
Editable (open) insets are entered and the cursor move detects word boundaries inside them.