Draft: Added start offset handle for TextPath items

This commit introduces the on-canvas handle to adjust the startOffset attribute of TextPath SVG elements.

Latest_Iteration

Delivers:

ux#256

Tasks

• Implement start, end and middle handles to move text on path
• Use more appropriate handle shapes
• Implement text on path popover
  • Start offset
  • Offset on normal
  • Flip buttons
  • Icons
• Flip sides by dragging the middle handle
• Status bar messages and hints
• Fix performance
• Change cursor shape on hovering over the handles
  • start and end
  • Middle flipper
• Update the handle positions as soon as startOffset changes