Improved Titles options in Plots subdialog

[Vitalis95]

Fixes #8477
@rdstern , @N-thony , @lloyddewit , this is ready for review.

[rdstern]

@Vitalis95 nicely done. This was perfect. This is the second different edit to the code that I am approving first time round.
@lloyddewit just to report something very nice also about your script windows.
a) I did the check by first sending the script without a title to a script window.
b) Then I made a new empty script window and added a title. It then adds a whole set of titles to the script.
c) Then another script window and took away the title - to see if Vitalis also took it away from the code. He did.

Now "your" script window properties. This is important, because the graphs are a high-spot of R and are very logical, but also produce very long commands. They are obvious bits of code to get the script and tweak. I have been doing that and finding it a bit awkward with the single line being so long. So I tried here and found:

a) It indents progressively. Very nice.
b) Undo and Redo also both work really well. I can easily get back to the long line if I wish - even 5 progressive undos.

My big idea for Antoine is for a sort of primitive undo - but very nice - for R-Instat as a whole. How nice that I am now discovering it is working so sweetly in the script windows!