Added configurable selectionbox width. Min width = 1, Max = 5

This PR adds an option to configure the width of the selectionbox lines.
Former default was 3, current default is 2. Max is 5, Min is 1. (Minecraft width is about 1)

*For those who think Minetest's default selectionbox is too garish (or not garish enough.)*

**Setting (minetest.conf.example):**
```
# Width of the selectionbox's lines (Between 1 and 5)
#selectionbox_width = 2
```