Just show the actual expression. We need to do a bit of mangling
here as wx has no way to turn off mnemonics parsing, so do that
as well.

If an expression can't be parsed normally, we then look to see if it's a
simple device name. This keeps backwards compatibility with simple input
ocnfigurations, where people just used the Detect button.

The full expression is quite often too big for a simple button
label, so encourage people to use the full editor to edit it.

This contains a new, hand-written expression parser to replace the old
hack language based on string munging. The new approach is a simple
AST-based evaluation approach, instead of the "list of operations"
infix-based hack that there was before.

The new language for configuration has support for parentheses, and
counts "!" as a unary operator instead of the binary "NOT OR" operator
it was before. A simple example:

  (X & Y) | !B

Explicit device references, and complex device names ("Right Y+") are
handled with backticks and colons:

  (`SDL/0/6 axis joystick:Right X+` & `DInput/0/Keyboard Mouse:A`)

The basic editor UI that inserts tokens has not been updated to reflect
the new language.