[LyX/master] Allow compiling with Qt6

[Enrico Forestieri]

On Mon, Mar 15, 2021 at 10:52:19PM -0400, Scott Kostyshak wrote:
> 
> Are there still QRegExp that need to be changed? I got an error from
> src/support/qstring_helpers.cpp.

I actually don't know. Let me explain. There should be a compatibility
layer for using Qt5 constructs, so I didn't search for all occurrences
of QRegExp's but only changed the places where I was getting compiling
errors. The code in qstring_helpers.cpp compiled without issues for me
on both Linux and Windows, and thus I did not touch it.
What kind of error are you getting?

> Also, why use
> 
>   #include <QtCore5Compat/QRegExp>
> 
> in CategorizedCombo.cpp, but not other places?

Because it was the only place where I had to explicitly include it.

-- 
Enrico