Add bind to Insert > Separated <Environment> Above

The binding <Alt + p, Shift + Return> is already used for
environment-split outer, so we bind <Alt + p, Alt + Return>
to environment-split before.

(cherry picked from commit 386aba099b)

lib/bind/menus.bind

@@ -126,6 +126,7 @@ Format 4
 \bind "M-p Up"			"outline-up"
 \bind "M-p Down"		"outline-down"
 
+\bind "M-p M-Return"		"environment-split before"
 \bind "M-p Return"		"command-alternatives environment-split ; environment-split previous"
 \bind "M-p S-Return"		"environment-split outer"
 

status.23x

@@ -22,6 +22,8 @@ What's new
 
 - Keep citation style when all selected citations are deleted (bug 11748).
 
+- Add shortcut for Insert > Separated <Environment> Above.
+
 
 * MISCELLANEOUS