Request: manually renumber linedefs

[kitchen-ace]

SLADE Version: 3.2.0. b2 Operating System: Linux

Issue Details: It would be nice if you could easily renumber linedefs, as some vanilla tricks require certain linedefs to be the lowest number, not to mention the numeric model for triggers. You can usually accomplish this by adding and deleting vertices, but it seems to work inconsistently and is kind of a pain.

Something like selecting two linedefs and swapping their number would be great. Selecting a group of linedefs and then being able to specify which one should be the lowest number would be even better.