Remember preview line on backspace (Pen tool fix inkscape/282)

For the extra case: when backspacing while in spiro mode, then switching back to normal mode, the preview line disappears.

Fixes #282 (closed)

See screen

Note the extra case is required because for spiro/bspline modes, which have constant redraws, flattening green_bpaths.

Out of scope, but to eliminate the excessive redraws, the offending function is PenTool::_bsplineSpiroMotion and the extra case to consider is Alt+move (requires the green_bpaths to be updated, without visual glitches)

Update: setting WIP status till I remember how to replicate this problem 🤦 Geez, such an edge case. Here's a video. Note that if you touch the canvas between backspace and mode change, this doesn't occur.

Update (2): more accurate commit message, clearer code.