add english names of variable roles to the labels

using labels for possibility of translating variable role names.

translations/en/LC_MESSAGES/messages.po

@@ -397,7 +397,7 @@ msgid "connect_guest_teacher"
 msgstr "I would like to be connected with a guest teacher who can give a few lessons"
 
 msgid "constant_variable_role"
-msgstr ""
+msgstr "constant"
 
 msgid "content_invalid"
 msgstr "This adventure is invalid."
@@ -820,7 +820,7 @@ msgid "input"
 msgstr "input from `{ask}`"
 
 msgid "input_variable_role"
-msgstr ""
+msgstr "input"
 
 msgid "integer"
 msgstr "a number"
@@ -946,7 +946,7 @@ msgid "list"
 msgstr "a list"
 
 msgid "list_variable_role"
-msgstr ""
+msgstr "list"
 
 msgid "live_dashboard"
 msgstr "Live Dashboard"
@@ -1628,7 +1628,7 @@ msgid "step_title"
 msgstr "Assignment"
 
 msgid "stepper_variable_role"
-msgstr ""
+msgstr "stepper"
 
 msgid "stop_code_button"
 msgstr "Stop program"
@@ -1879,7 +1879,7 @@ msgid "unique_usernames"
 msgstr "All usernames need to be unique."
 
 msgid "unknown_variable_role"
-msgstr ""
+msgstr "unknown"
 
 msgid "unlock_thresholds"
 msgstr "Unlock level thresholds"
@@ -1957,7 +1957,7 @@ msgid "view_slides"
 msgstr "View slides"
 
 msgid "walker_variable_role"
-msgstr ""
+msgstr "walker"
 
 msgid "welcome"
 msgstr "Welcome"
@@ -2191,9 +2191,4 @@ msgstr "Your program"
 #~ msgid "variables"
 #~ msgstr "Variables"
 
-#~ msgid "container_variable_role"
-#~ msgstr ""
-
-#~ msgid "input_constant_variable_role"
-#~ msgstr ""