This reverts commit cce809ac90cc195f0c88a6cc788b3ff7359083a5.

The code was actually correct: "expr" is never allocated when an error is
returned. This means when the expression parser fails, deleting "expr" means
deleting an uninitialized pointer.

Because there's a return here, expr should be deleted since it's not assigned to anything before returning.

Without clearing out the "accumulator" for the backtick parsing,
our control name was full of junk (the previous device name) causing
us to not correctly find the control.

Ensure that always we clear the "accumulator" string during backtick
parsing.

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.

Otherwise, valid control names like "Cursor X+" would be incorrectly
tokenized as "`Cursor` `X` +", causing the parser to first abort trying to
find a control named `Cursor` rather than aborting with invalid syntax on
the bad binop.

We could also do this by resolving devices lazily, but since simple
control name bindings are going to be 90% of usecases, just look for these
first.

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.

We need to make sure we eat the delimiter, otherwise we'll notice
the colon / backtick and think it's either a new control or part
of the control name

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