Add search function for favorites

[fkztw]

After I added lots of sites into favorites, it's hard to find a setting of a specific site by just using scroll bar.
Maybe add a search function for favorites just like the same in the Site List would be a better UX.
Actually, press <ctrl> + f while viewing the Favorites list will invoke a simple search bar for users. (might be the GTK built-in function?)
If it's not that necessary to implement this emphasizing function, maybe can just add a notice like press ctrl+f to search in the Favorites list.

[kidwm]

我以為年輕人都只要開 ptt.cc 跟 bs2.to 就夠了耶 XD