Add functionality to minimap flags system (#931)

- Add parameter 'temporary' to method 'UIMinimap:addFlag', who prevents saving flag with this parameter (look 'UIMinimap:save' method), add flag only at game session time. Old parameters list work the same at previous - backward compatibility.
- Add possibility to add icon from outside (for example from module folder) using link to this file. Old system with choose flag name by number work the same at previous, now method check if paramether is number then use old system otherwise get icon from path - backward compatibility. 'Mods' friendly.

modules/gamelib/ui/uiminimap.lua

@@ -72,12 +72,14 @@ end
 function UIMinimap:save()
   local settings = { flags={} }
   for _,flag in pairs(self.flags) do
+    if not flag.temporary then
       table.insert(settings.flags, {
         position = flag.pos,
         icon = flag.icon,
         description = flag.description,
       })
     end
+  end
   settings.zoom = self:getZoom()
   g_settings.setNode('Minimap', settings)
 end
@@ -110,19 +112,25 @@ function UIMinimap:setCrossPosition(pos)
   end
 end
 
-function UIMinimap:addFlag(pos, icon, description)
+function UIMinimap:addFlag(pos, icon, description, temporary)
   if not pos or not icon then return end
   local flag = self:getFlag(pos, icon, description)
   if flag or not icon then
     return
   end
+  temporary = temporary or false
 
   flag = g_ui.createWidget('MinimapFlag')
   self:insertChild(1, flag)
   flag.pos = pos
   flag.description = description
   flag.icon = icon
+  flag.temporary = temporary
+  if type(tonumber(icon)) == 'number' then
     flag:setIcon('/images/game/minimap/flag' .. icon)
+  else
+    flag:setIcon(resolvepath(icon, 1))
+  end
   flag:setTooltip(description)
   flag.onMouseRelease = onFlagMouseRelease
   flag.onDestroy = function() table.removevalue(self.flags, flag) end