Add formspec toolkit and refactor mainmenu to use it

Fix crash on using cursor keys in client menu without selected server
Add support for non fixed size tabviews

builtin/mainmenu/dlg_delete_world.lua

@@ -0,0 +1,64 @@
+--Minetest
+--Copyright (C) 2014 sapier
+--
+--This program is free software; you can redistribute it and/or modify
+--it under the terms of the GNU Lesser General Public License as published by
+--the Free Software Foundation; either version 2.1 of the License, or
+--(at your option) any later version.
+--
+--This program is distributed in the hope that it will be useful,
+--but WITHOUT ANY WARRANTY; without even the implied warranty of
+--MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--GNU Lesser General Public License for more details.
+--
+--You should have received a copy of the GNU Lesser General Public License along
+--with this program; if not, write to the Free Software Foundation, Inc.,
+--51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+
+local function create_world_formspec(dialogdata)
+
+	local retval =
+		"size[12,6,true]" ..
+		"label[2,2;" ..
+		fgettext("Delete World \"$1\"?", dialogdata.delete_name) .. "]"..
+		"button[3.5,4.2;2.6,0.5;world_delete_confirm;" .. fgettext("Yes").. "]" ..
+		"button[6,4.2;2.8,0.5;world_delete_cancel;" .. fgettext("No") .. "]"
+	return retval
+end
+
+local function create_world_buttonhandler(this, fields)
+	if fields["world_delete_confirm"] then
+
+		if this.data.delete_index > 0 and
+			this.data.delete_index <= #menudata.worldlist:get_raw_list() then
+			core.delete_world(this.data.delete_index)
+			menudata.worldlist:refresh()
+		end
+		this:delete()
+		return true
+	end
+	
+	if fields["world_delete_cancel"] then
+		this:delete()
+		return true
+	end
+	
+	return false
+end
+
+
+function create_delete_world_dlg(name_to_del,index_to_del)
+
+	assert(name_to_del ~= nil and type(name_to_del) == "string" and name_to_del ~= "")
+	assert(index_to_del ~= nil and type(index_to_del) == "number")
+
+	local retval = dialog_create("sp_create_world",
+					create_world_formspec,
+					create_world_buttonhandler,
+					nil)
+	retval.data.delete_name  = name_to_del
+	retval.data.delete_index = index_to_del
+	
+	return retval
+end