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/fstk/dialog.lua

@@ -0,0 +1,69 @@
+--Minetest
+--Copyright (C) 2014 sapier
+--
+--self 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.
+--
+--self 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 self program; if not, write to the Free Software Foundation, Inc.,
+--51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+local function dialog_event_handler(self,event)
+	if self.user_eventhandler == nil or
+		self.user_eventhandler(event) == false then
+
+		--close dialog on esc
+		if event == "MenuQuit" then
+			self:delete()
+			return true
+		end
+	end
+end
+
+local dialog_metatable = {
+	eventhandler = dialog_event_handler,
+	get_formspec = function(self)
+				if not self.hidden then return self.formspec(self.data) end
+			end,
+	handle_buttons = function(self,fields)
+				if not self.hidden then return self.buttonhandler(self,fields) end
+			end,
+	handle_events  = function(self,event)
+				if not self.hidden then return self.eventhandler(self,event) end
+			end,
+	hide = function(self) self.hidden = true end,
+	show = function(self) self.hidden = false end,
+	delete = function(self)
+			if self.parent ~= nil then
+				self.parent:show()
+			end
+			ui.delete(self)
+		end,
+	set_parent = function(self,parent) self.parent = parent end
+}
+dialog_metatable.__index = dialog_metatable
+
+function dialog_create(name,get_formspec,buttonhandler,eventhandler)
+	local self = {}
+
+	self.name = name
+	self.type = "toplevel"
+	self.hidden = true
+	self.data = {}
+
+	self.formspec      = get_formspec
+	self.buttonhandler = buttonhandler
+	self.user_eventhandler  = eventhandler
+	
+	setmetatable(self,dialog_metatable)
+
+	ui.add(self)
+	return self
+end