Sticky extensions

[fregante]

This is something I've wanted for a while, but I never found a way to add such option without adding yet another icon to the right.

Since this is a "configuration" rather than an "action" on the extension, it probably shouldn't be added to the list.

That means it probably needs to be in the options page. Since <select multiple> is junk, that leaves us with checkboxes.

PR welcome. It would be great to use a Svelte component to generate the list, then OptionsSync will automatically pick up and store the "checked" checkboxes: <input type="checkbox" name="sticky" value={extensionId}>