Improve custom header offset text box

[victorbush]

Opening in "Custom" mode: the text box for the header offset needs improved. It doesn't support hex values (must be decimal). It should also trim the beginning and end of string for non-digit characters. There's a weird thing if you copy from the Windows calculator app, the string has some whitespace characters in front and at the end (but you can't see it when pasted into the text box). So you always get an "invalid offset".

Would like to be able to have 0x prefix for hex values. Without the prefix, should be interpreted as decimal.