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>
Merge pull request #497 from marius-avram/save_load_script

Saving and loading expression scripts