Fix sometimes select plugin setting from context menu not works problem.

[ulion]

currently, when we are in a plugin source dir, if selected item's path is not a plugin/script path, e.g. raw final http url, select 'Plugin Settings' from the item's context menu items will now show the plugin's setting dialog.
that's because the current code only use the selected item's path's hostname to get plugin/addon, it will fail in condition I mentioned.
I checked codes to show the context menu, there's condition like this:

    if (item->IsPlugin() || item->IsScript() || m_vecItems->IsPlugin())
      buttons.Add(CONTEXT_BUTTON_PLUGIN_SETTINGS, 1045);

obviously we will lost if the item is an http url item in a plugin dir in current code, so here's the fix PR.

[jmarshallnz]

Looks fine, though agreed with t-nelson that it could be made clearer.

[ulion]

added temp variable and comment as @t-nelson suggested.

[ulion]

if no objection, I will merge this after 2 days.