Today I fixed the bugs in new line creation and sent a new pull request here.
The lines are now visible during their creation.
The first point is stored in the LineOperations class, and the second one is replaced by current cursor position each time the class receive the mouseMove event. The line is stored in the TempEntities class, and moved to document when a click is received.

I also added the creation of circles and arcs with the same model as lines, in this pull request.

Here is a screenshot of new entities created with LibreCAD 3: