Add a button in the toolbar to reset the editor

[corebreaker]

Fixes the issue #323 .

It adds a button to reset the content of the editor. The session storage is also updated.

[shepmaster]

Oh wow, I never responded to this, and for that I apologize!

I don't believe that this action is common enough to deserve space on the toolbar, which I attempt to keep minimal to avoid overwhelming people.

My vision for this functionality is two-fold:

1. Place a "reset text" (and potentially "reset all settings") button in the configuration menu.
2. Track when the user last visited and if it's long enough ago (exact amount TBD), pop up an offering to reset the text / settings.

[CleanCut]

Huh. I thought it was really weird that there wasn't a reset button right about there. I thought it was a UI bug or something, actually. I would expect there to be some easily-findable way to reset the state.