Split tag reading and tag handling logic

[TheNailDev]

Hello there,
I saw the comment in #789 from @barbeque-squared about how the current tag reading and evaluation in USong.pas should be changed to allow future extensions, especially with regard to handling different txt versions in the future.
Based of the comment I changed the ReadTXTHeader function to first read all tags into a TFPGMap and afterward evaluate the tags from the map.
I hope this can be a good starting point to allow different tag handling logic depending on the txt version.