Revert "toc bind: 'dialog-show' -> 'dialog-toggle' (#8388)"

This reverts commit bede4d320b.

As JMarc notes [1], if we replaced show with toggle, we would lose some
functionality:

  There is one use that is not possible with this change (as I argued
  before), which is a fully keyboard-based navigation.

  With LyX 2.1, I can do C-M-o and navigate with cursor in the
  outline. I can do the same with advanced S&R. IN this sense,
  toggling is a step back IMO.

The plan now is to find another way to close the dialog (without
using toggle), such as having ESC close it or another key bind.

Ticket #8388 will stay closed because 'show' and 'toggle' are too
close for the cost of having two separate bindings.

[1] http://www.mail-archive.com/lyx-devel@lists.lyx.org/msg187693.html

lib/bind/cua.bind

@@ -120,7 +120,7 @@ Format 2
 \bind "S-F7"			"thesaurus-entry"
 
 \bind "M-x"			"command-execute"
-\bind "C-M-o"			"dialog-toggle toc"
+\bind "C-M-o"			"dialog-show toc"
 
 \bind "F11"			"ui-toggle fullscreen"
 

lib/bind/emacs.bind

@@ -144,7 +144,7 @@ Format 2
 #bind "F10"                    "------"
 
 \bind "M-x"			"command-execute"
-\bind "C-M-o"			"dialog-toggle toc"
+\bind "C-M-o"			"dialog-show toc"
 
 \bind "M-equal"			"buffer-zoom-in"
 \bind "M-plus"			"buffer-zoom-in"

lib/bind/mac.bind

@@ -280,7 +280,7 @@ Format 2
 \bind "F7"                           "dialog-show spellchecker"
 \bind "S-F7"                         "thesaurus-entry"
 
-\bind "C-M-o"                        "dialog-toggle toc"
+\bind "C-M-o"                        "dialog-show toc"
 
 #
 # Additional Motion group