Fix shift + tab not focusing search engine dropdown while on search bar

[Zharkan]

fix #14126

[mr-cheffy]

I feel like this would be better suited to be submitted to firefox? Would off-load work from us: https://firefox-source-docs.mozilla.org/contributing/how_to_submit_a_patch.html

[Zharkan]

This is Zen specific though, upstream Firefox has scotchBonnet.enableOverride enabled by default, so Tab/Shift+Tab navigation to the search engine dropdown already works there. The issue only exists because Zen disables scotchBonnet, which gates that code path. The search bar widget still uses SearchModeSwitcher regardless of scotchBonnet, so we need this patch to wire up the Tab navigation for it on our side, right ?