Add paste-mode to allow multiline text pasting

Ctrl-T toggles paste mode.
Useful when pasting multiline text because it avoids triggering an
enter keypress after linebreaks.
Instead, every \r is translated to \n which is again translated to a
pilcrow (see previous commit).
The pasted text can then be sent as a whole instead of splitting up in
multiple messages.

src/windows.h

@@ -237,6 +237,7 @@ struct ChatContext {
 #endif
 
     uint8_t self_is_typing;
+    uint8_t pastemode; /* whether to translate \r to \n */
 
     WINDOW *history;
     WINDOW *linewin;