[lmi] "What's this" help for text controls

[Vadim Zeitlin]

 As Greg has asked in private mail yesterday, we should decide how should
this "What's this" popup menu combine with the standard native text control
popup menu. Looking at a couple of different applications and Windows
standard dialogs, it seems that the rule is quite simple: for readonly text
controls, the "What's this" menu is shown (logical as the right click menu
isn't very useful for them, the only valid item in it for readonly control
is "Copy" and this can be done using Ctrl-C too). But for normal controls,
even in a dialog with context-sensitive help, the standard menu with text
operations is shown. So I propose to do the same in lmi -- please let me
know if there are any objections.

 Another tiny proposal: I think it only makes sense to show "What's this"
help in a dialog/frame with wxDIALOG_EX_CONTEXTHELP style. This is both
because if we have the context-sensitive help anyhow, we should also allow
showing it via "?" button in the title bar and because, vice versa, without
this "?" button the user might not even realize that there is
context-sensitive help available. Again, please let me know if you disagree
with this, if not we're going to modify the code accordingly.

 Thanks!
VZ