Add GUI configuration for choice of clipboards.

On all platforms, you can now configure which clipboard the mouse
pastes from, which clipboard Ctrl-Ins and Shift-Ins access, and which
Ctrl-Shift-C and Ctrl-Shift-V access. In each case, the options are:

 - nothing at all
 - a clipboard which is implicitly written by the act of mouse
   selection (the PRIMARY selection on X, CLIP_LOCAL everywhere else)
 - the standard clipboard written by explicit copy/paste UI actions
   (CLIPBOARD on X, the unique system clipboard elsewhere).

Also, you can control whether selecting text with the mouse _also_
writes to the explicitly accessed clipboard.

The wording of the various messages changes between platforms, but the
basic UI shape is the same everywhere.

doc/config.but

@@ -1469,6 +1469,68 @@ select a rectangular block. Using the \q{Default selection mode}
 control, you can set \i{rectangular selection} as the default, and then
 you have to hold down Alt to get the \e{normal} behaviour.
 
+\S{config-clipboards} Assigning copy and paste actions to clipboards
+
+Here you can configure which clipboard(s) are written or read by
+PuTTY's various copy and paste actions.
+
+The X Window System provides multiple clipboards (or \q{selections}),
+and many applications support more than one of them by a different
+user interface mechanism.
+
+The two most commonly used selections are called \cq{PRIMARY} and
+\cq{CLIPBOARD}; in applications supporting both, the usual behaviour
+is that \cw{PRIMARY} is used by mouse-only actions (selecting text
+automatically copies it to \cw{PRIMARY}, and middle-clicking pastes
+from \cw{PRIMARY}), whereas \cw{CLIPBOARD} is used by explicit Copy
+and Paste menu items or keypresses such as Ctrl-C and Ctrl-V.
+
+On other platforms, where there is a single system clipboard, PuTTY
+provides a second clipboard-like facility by permitting you to paste
+the text you last selected in \e{this window}, whether or not it is
+currently also in the system clipboard.
+
+\S2{config-selection-autocopy} \q{Auto-copy selected text}
+
+\cfg{winhelp-topic}{selection.autocopy}
+
+The checkbox \q{Auto-copy selected text to system clipboard} controls
+whether or not selecting text in the PuTTY terminal window
+automatically has the side effect of copying it to the system
+clipboard, without requiring a separate user interface action.
+
+On X, the wording of this option is changed slightly so that
+\cq{CLIPBOARD} is mentioned in place of the \q{system clipboard}. Text
+selected in the terminal window will \e{always} be automatically
+placed in the \cw{PRIMARY} selection, but if you tick this box, it
+will \e{also} be placed in \cq{CLIPBOARD} at the same time.
+
+\S2{config-selection-clipactions} Choosing a clipboard for UI actions
+
+\cfg{winhelp-topic}{selection.clipactions}
+
+PuTTY has three user-interface actions which can be configured to
+paste into the terminal (not counting menu items). You can click
+whichever mouse button (if any) is configured to paste (see
+\k{config-mouse}); you can press Shift-Ins; or you can press
+Ctrl-Shift-V, although that action is not enabled by default.
+
+You can configure which of the available clipboards each of these
+actions pastes from (including turning the paste action off
+completely). On platforms with a single system clipboard, the
+available options are to paste from that clipboard or to paste from
+PuTTY's internal memory of the last selected text within that window.
+On X, the options are \cw{CLIPBOARD} or \cw{PRIMARY}.
+
+(\cw{PRIMARY} is conceptually similar in that it \e{also} refers to
+the last selected text \dash just across all applications instead of
+just this window.)
+
+The two keyboard options each come with a corresponding key to copy
+\e{to} the same clipboard. Whatever you configure Shift-Ins to paste
+from, Ctrl-Ins will copy to the same location; similarly, Ctrl-Shift-C
+will copy to whatever Ctrl-Shift-V pastes from.
+
 \H{config-selection-words} The Words panel
 
 PuTTY will \I{word-by-word selection}select a word at a time in the