Reduce path editing clutter with options to hide non-selected anchors' handles

[Keavon]

There should be three modes, explained below. But in summary:

• The first mode is the current behavior where all handles are shown.
• The second mode is where all selected anchors' segments' handles are shown.
• The third mode is where only the frontier handles are shown.

Pen tool

In the Pen tool, only these circled handles should appear:

• Only the last-drawn anchor and its handles are shown, plus the handle on the other end of its connected segment. The Pen tool only has modes 1 and 2, but not 3.

  

Path tool

In the Path tool, which allows multi-selection of anchors, these are the cases where it should appear:

• When one anchor is selected, its handles should be shown, as well as the handles of its connected segment(s). When one anchor is selected, this same situation applies to both modes 2 and 3, because mode 3 only becomes active with multi-selection (otherwise mode 2 is used).

  

• Mode 2: When two or more anchors are selected, all handles (circled in red) on either end of the adjacent segments (purple) are shown.

  

• Mode 3: When two or more anchors are selected, only the handles on the frontier are shown. The green shows the adjacent segments to the selected anchors, and the ends of the green are the frontiers. Selected in red are the only handles to be shown.

  

Remember: only the handles in the diagrams above which are circled in red should appear, all other handles should be hidden.

The three modes (two for Pen tool) should be in the respective tools' control bars as a radio button. just use "1", "2", and "3" text for now until I come up with names or icons. The control bar is the area on the left above the drawing viewport where the tool controls are located.

[4adex]

I am working on this, opened a draft PR #2264