Add reset subcommand for sidebar

[ernilambar]

Fixes #3063

[danielbachhuber]

For this command, can you create a separate features/sidebar-reset.feature file and move your tests in there?

[danielbachhuber]

This should probably be "Reset WordPress sidebars"

[danielbachhuber]

@ernilambar I just realized this uses a lot of code copied from class Widget_Command. Could we make this wp widget reset or similar, as to reuse the code?

[ernilambar]

@danielbachhuber I also felt that when I had to copy functions from Widget command. I even had this crazy idea of using launch_self for some tasks, but on searching I could only found two places where it is used now. So, I assumed it is best to avoid in core commands.

Since this is basically related to sidebar, I tried to keep it under sidebar command. Adding it under widget is also one approach I guess.

wp widget reset
Are we up for this?

1. wp widget reset sidebar-1
2. wp widget reset sidebar-1 sidebar-2
3. wp widget reset --all
4. wp widget reset [This should also throw error I believe. Like we do in plugin update - Error: Please specify one or more plugins, or use --all. ]
5. wp widget reset wp_inactive_widgets [Should throw error.]

Re 5: There is button Clean Inactive Widgets in the /wp-admin/widgets.php. This functionality could be incorporated for command in 5. Or may be we should implement separately creating separate ticket?