#598 Trimming line does not un-highlight after done trimming

[mlt]

For the new RC 2.2.0 of LibreCad, after completing a trimming operation, the trimming line does not un-hilite until you zoom.

Steps to reproduce:

1. Draw 2 intersecting lines
2. Type 'tm' to trim
3. Select one of the lines as the trimming line
4. Click the other line to trim
5. Press 'esc' to get out of trim operation
   Result: trimming line is still highlighted green.
   Should be: either ask to set a new trimming line, or unhighlight line.

Self-compiled MacOS build:
Version: 2.2.0-rc1-14-g0559d309
Compiler: Clang 9.1.0
Compiled on: Apr 8 2018
Qt Version: 5.10.1
Boost Version: 1.65.1
System: macOS High Sierra (10.13)