plug-ins: add name label for brush pipe selection modes

Previously selection mode values were shown as-is with their name (e.g. "xtilt") in a drop-down in the brush pipe export dialog, with no support for translation.

This MR adds a label so we can have nicer strings for the GUI (e.g "xtilt" -> "X tilt") and also makes them translatable. The names that were previously shown are variables in the .gih file header, so they can't be changed.

Above: drop-down contents after this MR.