Indentation Improvements

[baumths]

• Add a way to modify the tree lines path object before drawing on the canvas (enabling line dashing/dotting).

  The actual line dashing algorithm was not implemented in this PR, but the demo app was updated to include a visual example of the above using the path_drawing package.

• IndentGuide.padding

  Optional inset values added to the padding that indents tree nodes and wraps TreeIndentation.child.

• Add an optional ┐ shaped line to ConnectingLinesGuide which connects parent and descendant lines.

  the above was pointed out in Nodes with a higher content than normal (eg. multi-line text) #29 (comment)

Resolves #49