Merge Plugins submenu entries into Tools

Now that we have a Tools 1st-level menu, having a separate top-level menu entry for Plugins is wasteful, especially for entries that are much less frequently used than for example the average entry under Tools

I recommend we move the two Plugin Manager/Creator menu entries from Plugins to Tools, and remove the 1st-level Plugins menu entry altogether. My suggestion for the location of the Plugin Manager/Creator menus is at the bottom of the tools menu.