numberbox behaves more sanily when used backspace/delete in it

game/engines/default/engine/ui/Numberbox.lua

@@ -50,11 +50,11 @@ function _M:generate()
 	self.key:addIgnore("_DOWN", false)
 
 	self.key:addCommands{
-		_UP = function() self.first = false self:updateText(1) end,
-		_DOWN = function() self.first = false self:updateText(-1) end,
+		_UP = function() self:updateText(1) end,
+		_DOWN = function() self:updateText(-1) end,
 		__TEXTINPUT = function(c)
 			if self.first then self.first = false self.tmp = {} self.cursor = 1 end
-			if #self.tmp and (c == '-' or c == '0' or c == '1' or c == '2' or c == '3' or c == '4' or c == '5' or c == '6' or c == '7' or c == '8' or c == '9') then
+			if #self.tmp and ((self.cursor == 1 and c == '-') or (c >= '0' and c <= '9')) then
 				table.insert(self.tmp, self.cursor, c)
 				self.cursor = self.cursor + 1
 				self.scroll = util.scroll(self.cursor, self.scroll, self.max_display)
@@ -65,6 +65,7 @@ function _M:generate()
 end
 
 function _M:updateText(v)
+	self.first = false
 	local old = self.number
 	if not v then
 		self.number = self.tmp and tonumber(table.concat(self.tmp)) or 0