-add a way to reset their value (from the mappings UI)
-fix "memory leak" where they would never be cleaned,
one would be created every time you wrote a character after a "$"
-fix ability to create variables with an empty string by just writing "$" (+added error for it)
-Add $ operator to the UI operators list, to expose this functionality even more
Fixes the expression window being spammed with the first entry in the
Operators or Functions select menus when scrolling the mouse wheel while
hovering over them.
Fixes https://bugs.dolphin-emu.org/issues/12405
