Changed default image viewer helper message to reflect current controls.

[toastisme]

This just updates the default image viewer helper message, as it seems to be out of date with current controls.

I can't find anywhere where the intensity plot is used (I'm guessing it's OnShowPlot), and the right click to zoom in wx_viewer doesn't seem to be in use.

[codecov]

Codecov Report

Merging #1869 (8893825) into main (9aede7c) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1869      +/-   ##
==========================================
- Coverage   67.15%   67.05%   -0.11%     
==========================================
  Files         617      617              
  Lines       69532    70303     +771     
  Branches     9672     9886     +214     
==========================================
+ Hits        46697    47144     +447     
- Misses      20892    21217     +325     
+ Partials     1943     1942       -1     

[rjgildea]

Are both of these actually required or (more likely) does one of them represent dead code. in which case it should probably be removed (not suggesting you should do that here in this PR)?

[toastisme]

Are both of these actually required or (more likely) does one of them represent dead code. in which case it should probably be removed (not suggesting you should do that here in this PR)?

It looks like only frame.py is used, but felt it was best to at least keep rstbx_frame.py updated in case this is used in some other context.