Text on path improvements

Add on-canvas controls to adjust startOffset

UX proposal: ux#256

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

Clicking the start offset knots opens the TextpathPopover, allowing adjustments to:

• startOffset
• side

Latest Iteration

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
• A guide line for offset from the normal
• A right click menu item to select the parent path
• Add an arrow for the path direction to the Flip icon