Windows class:
Added 'CreateShowColumnsPopupMenu()' and 'SaveColumnsDisplay()' functions

MusicTable class:
Added 'show(int)' function
Modified 'hide(int)' from private to public

Other stuff:
Added a .txt file in the resources folder to save column display settings

Summary:
Columns can now be displayed/hidden via the right clicking the column titles.
Column display statuses are saved.

Problems:
If you move the columns around, it messes it all up. Might look into this (if it's a testing req)
Fix problem in ShiTunes.updateAllWindows() method.
Persistent column config: using db instead of file, index issues remain causing erratic functionality.
Persistent column configuration implemented correctly (visibility only).