Enable "Track changes" from the toolbar

[Daniel]

Currently, it is really cumbersome to enable "Track changes" from the 
toolbar since it is impossible to start "Track changes" without changing 
the toolbar visibility settings.

What one has to do in order to enable "Track changes" from the toolbar 
(and not make any changes to the toolbar visibility):

1) Click on the "Show review toolbar" button (not exactly aptly named)
2) Set the visibility of the Review toolbar to "On"
3) Enable "Track changes"
4) Click on the "Show review toolbar" button
5) Set the visibility of the Review toolbar to "Automatic"

IMO, this is way to cumbersome and easy to forget to change back the 
visibility of the toolbar. Instead, I suggest that there should be just 
a button to turn "Track changes" on. This will also show the toolbar 
(already implemented).

Bug: https://www.lyx.org/trac/ticket/12696

Daniel