Allow the Trim/Extend Tool to keep the reference line for multiple trim/extend actions

[MattiasSp]

Feature description

Trimming or extending a number of lines toward a single reference line would become a lot more efficient if the reference line could stick around as one clicks on all the target lines to trim/extend. For example, holding the Shift or Ctrl key while clicking on target lines to trim/extend could let the same reference line be used immediately for the next click.