Search Widget standardization

[pajlada]

We're starting to get a few more search inputs around Chatterino, it would be good to standardize their behaviour through a SearchInputWidget of some kind

1. When the Search Input is selected and is not empty, pressing Escape should be intercepted and clear the search entry
2. The clear button behaviour of the input should be enabled (See Add clear button to settings search bar #3403)
3. The SearchInputWidget must force the developer to specify a placeholder text

CC @zneix

[Mm2PL]

It would also be nice if it forced the developer to use the hotkeys system and add a search action.