Query before destroying important structures (rail stations and industries)

Prior to this change, the use of NewObjects often lead to the problem of players trying to clear those and accidentally destroying a rail station or (when using magic bulldozer) an industry. This action cannot be undone.

This change shows a query making sure the player actually wants to destroy the station or industry.

NOTE: The switch to a std::function no longer allows for the inequality check in ShowQuery in misc_gui. If this was required a different solution has to be found.

src/lang/english.txt

@@ -5550,6 +5550,10 @@ STR_ERROR_NAME_MUST_BE_UNIQUE                                   :{WHITE}Name mus
 STR_ERROR_GENERIC_OBJECT_IN_THE_WAY                             :{WHITE}{1:STRING} in the way
 STR_ERROR_NOT_ALLOWED_WHILE_PAUSED                              :{WHITE}Not allowed while paused
 
+# Clear area query
+STR_QUERY_CLEAR_AREA_CAPTION                                    :{WHITE}Clear area
+STR_CLEAR_AREA_CONFIRMATION_TEXT                                :{YELLOW}You are about to destroy an important structure. Are you sure?
+
 # Local authority errors
 STR_ERROR_LOCAL_AUTHORITY_REFUSES_TO_ALLOW_THIS                 :{WHITE}{TOWN} local authority refuses to allow this
 STR_ERROR_LOCAL_AUTHORITY_REFUSES_AIRPORT                       :{WHITE}{TOWN} local authority refuses to allow another airport to be built in this town