Add option to choose folding mode in outline pane

[melsophos]

It would be very useful to have a button in the outline pane to quickly change the folding mode: it could have "fold all", "expand all" and "expand to level n" (see #77).

[NeptuneProjects]

I strongly second this feature request. The outline is rather useless when I have to traverse scores of variables - I was hoping I could use it to navigate a long file quickly. Until "collapse all" is implemented, I will continue ignoring the outline altogether. Thank you for taking on this feature request.

[aminya]

I am designing a toolbar to allow executing some operations like revealing cursor, folding, etc.

You can follow the progress here:
#89