[WM] Deferr and get_current_main_content()

[jlm17]

While debugging this problem I have discovered that get_current_main_content() doesn't return the 
name of the main content item if that item is actually below the content item in which the call is 
made. For example if you have two content items like so:

<out name=something file=something else>
   ${some_perl_generated_content}
   ${main_content}
</out>

Then the call to get_current_main_content() in your ${some_perl_generated_content} returns the empty 
string. If you have this:

<out name=something file=something else>
   ${main_content}
   ${some_perl_generated_content}
</out>

Then the call to get_current_main_content() returns "main_content".

This problem is consistent; even the NavTree plugin suffers from this. Therefore you can have a 
navigation tree, but only if you want it in your html document somewhere AFTER the main content!