Paste figures with spaces should either cut or ignore spaces

[Hector B]

## Expected behaviour

Numbers with spaces as a thousand separator such as 123 456 copied from different sources should not generate errors.
Ideally spaces should be either ignored and preserved or cut making 123 456 -> 123456

## Actual behaviour

Numbers with spaces such as 123 456 copied from different sources produce errors "Malformed expression".
When spaces are manually deleted, the program successfully proceeds.

## Steps to reproduce the behaviour

    Start mate-calc.
    Copy digits with space as a thousand separator like 123 456.
    Paste them in the the program with Ctrl-V
    Add + 25 Enter
    Confirm error "Malformed expression".

https://github.com/mate-desktop/mate-calc/issues/207