Saving and loading expression scripts

Buttons handling this operations are found in the middle
between the top widgets and the input are where you can type
the expression.

Buttons are stretched to the whole width of the editor.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
You might want to print an error message in a message box if the file couldn't be open/created (use the `is_open()` method).