UI: Alllow for re-ordering third-party plugins' UIs

This won't be possible/easy until at least #1791 is merged.

Some users want to be able to control the ordering of plugins' UI in the main window.

Currently the order is determined by a sort on the plugin name string, as returned by `plugin_start3()`, else their folder name. This means a user can't just rename the plugin folders to achieve a desired order, they'd have to go in and edit any plugins that *do* return a name string.

With #1791 in place each plugin has its UI contained within a stably named Frame, so using `.grid(row=X)` to change where it appears will work. Obviously you'd want to re-row several plugins at once (at least a swap) to actually get a re-ordering.

What this needs is a clean UI, probably changing Settings > Plugins to allow for the re-ordering. Once that is achieved *then* some thought would have to be given to how to store this so it's persistent across restarts. If the user adds any new plugins then those could interfere and the user just gets to adjust things again.