Underline change tracking additions

[Daniel]

I saw that a new preference was added to disable Underline change 
tracking additions:

https://wiki.lyx.org/LyX/NewInLyX24#preferences

I just wanted to flag that this might make it harder to sensibly 
implement change tracking of some other changes, such as text style 
changes. For example, Libre Writer marks text style changes without 
underline. I think it would be nice to implement text style changes as 
Libre Writer does (minus making text bold which seems just an odd choice 
but I might be missing something because it seems so obviously wrong to 
me). (https://www.lyx.org/trac/ticket/1043#comment:21)

In contrast, the only application I came across without underlined 
change tracking additions is Apple Pages. However, it does not support 
tracking text style changes, as is currently the case in LyX as well.

Daniel