RF: Standardise functions between QT and WX dialog backends by getting both from a base class

[TEParsons]

This has the following advantages:

• Changing the behaviour of the Dlg class is now much easier for things like parsing syntax, where the backend doesn't matter, as you only have to change it in one place
• Reduces the danger of multiple backends slipping out of parity (as they already had a bit)
• If we wanted to add another backend (e.g. tkinter or any new lightweight GUI packages we come across) it's easy to get it up to the level qt and wx are at as most of the API is already written

[codecov]

Codecov Report

Merging #6335 (441ca09) into dev (4d8528c) will increase coverage by 0.18%.
Report is 16 commits behind head on dev.
The diff coverage is 61.98%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6335      +/-   ##
==========================================
+ Coverage   51.86%   52.05%   +0.18%     
==========================================
  Files         335      336       +1     
  Lines       61727    61661      -66     
==========================================
+ Hits        32015    32097      +82     
+ Misses      29712    29564     -148     

Components	Coverage Δ
app	∅ <ø> (∅)
boilerplate	∅ <ø> (∅)
library	∅ <ø> (∅)
vm-safe library	∅ <ø> (∅)