actions: move item selection to its own action and bind it to insert

doc/bindings

@@ -177,6 +177,9 @@
 #def_key "end"
 #  move_end
 #
+#def_key "insert"
+#  select_item
+#
 #def_key "space"
 #  press_space
 #
@@ -324,9 +327,6 @@
 #def_key "Y"
 #  toggle_replay_gain_mode
 #
-#def_key "t"
-#  toggle_space_mode
-#
 #def_key "T"
 #  toggle_add_mode
 #

doc/config

@@ -343,10 +343,6 @@
 ##
 #default_find_mode = wrapped
 #
-## Available values: add, select.
-##
-#default_space_mode = add
-#
 #default_tag_editor_pattern = %n - %t
 #
 #header_visibility = yes

doc/ncmpcpp.1

@@ -206,9 +206,6 @@ Default tag type for leftmost column in media library.
 .B default_find_mode = wrapped/normal
 If set to "wrapped", going from last found position to next will take you to the first one (same goes for the first position and going to previous one), otherwise no actions will be performed.
 .TP
-.B default_space_mode = select/add
-If set to "select", space will select items instead of adding them to playlist (although selecting by space is default and the only one action for space in Playlist).
-.TP
 .B default_tag_editor_pattern = TEXT
 Default pattern used by Tag editor's parser.
 .TP