Don't allow engine to add the implicit "Proceed" button

(this change makes it possible to theme the formspec)

init.lua

@@ -34,7 +34,12 @@ local tube_cbox = {
 -- the following functions based on the so-named ones in Jeija's digilines mod
 
 local reset_meta = function(pos)
-	minetest.get_meta(pos):set_string("formspec", "field[channel;Channel;${channel}]")
+	minetest.get_meta(pos):set_string("formspec",
+			"formspec_version[4]"..
+			"size[8,4]"..
+			"button_exit[3,2.5;2,0.5;proceed;Proceed]"..
+			"field[1.75,1.5;4.5,0.5;channel;Channel;${channel}]"
+	)
 end
 
 local on_digiline_receive_std = function(pos, node, channel, msg)
@@ -92,6 +97,7 @@ for _,tube in ipairs(nixie_types) do
 	local light2 = LIGHT_MAX-5
 	local description = S("Nixie Tube ("..tube..")")
 	local description2 = S("Decatron ("..tube..")")
+	local description3 = S("Numitron Tube")
 	local cathode = "nixie_tube_cathode_off.png^nixie_tube_cathode_"..tube..".png"
 	local cathode2 = "decatron_cathode_"..tube..".png"
 	local cathode3 = "numitron_filaments.png^numitron_"..tube..".png"
@@ -102,7 +108,6 @@ for _,tube in ipairs(nixie_types) do
 		light2 = nil
 		description = S("Nixie Tube")
 		description2 = S("Decatron")
-		description3 = S("Numitron Tube")
 		cathode = "nixie_tube_cathode_off.png"
 		cathode2 = "nixie_tube_blank.png"
 		cathode3 = "numitron_filaments.png"

mod.conf

@@ -1 +1,2 @@
 name = nixie_tubes
+min_minetest_version = 5.2.0