fixed append mode for LayoutPanel:multiRectSelect

[KlonZK]

does no longer overwrite old selectedItems table unscrupulously and updates selection status for newly selected items properly.

otherwise i may have broken the format by using tabs instead of spaces.

[gajop]

unbreak the whitespace format

[KlonZK]

i tried to use the github editor for this but it seems the whitespace settings there only affect display?

[gajop]

You might have to use a regular text editor