Fix visual glichy on multi-paste and make shortcut commands more coherent

[donho]

This PR is the following of #14422

This PR removes Shift-DEL, Ctrl-INS & Shift-INS shortcut from Scintilla.
Ref: #14401 (comment)

Now for Copy/Cut/Paste commands and their shortcut are coherent. ie. if user remaps the shortcuts of these 3 commands, both single/multiple selection operation with the commands in question will follow the changed shortcuts.

It also fixes 2 bugs:

1. visual glitch problem of read-only while multi-pasting.
2. the shortcut Ctrl-C in Search results works now.

Fix #14410

[donho]

@Yaron10

Using Ctrl+C or Ctrl+X in the Search Results window, the text in the edit zone is copied/cut.
See #11401 (comment).

The bugs are fixed now in this PR.
It fixes also the shortcut bug Ctrl-C not working in Search results.

[Yaron10]

It seems legit now.
(Just for the record: I still prefer Copy & Cut to be enabled/disabled as implemented in MS-Notepad, Firefox and many other applications).

Ctrl+C/X/V do not work in the Functions List's search box.
There might be other places I've missed. Also, it might affect some plugins.
Related: #13874.

If the Functions List's search box is not worth the extra code, finder->scintillaExecute(WM_COPY); might be removed as well.

[donho]

If the Functions List's search box is not worth the extra code, finder->scintillaExecute(WM_COPY); might be removed as well.

There are always the bugs in an application, and they all deserve to be fixed. We do our best to fix the bugs, and one less bug always makes the application better than it was before. So it doesn't make sense for me the above statement.

I will see what I can do about #13874 & FunctionList s search box.

Thank you anyway for your review.