The language learning website Linguno has a feature where letter accents are given number hotkeys when typing in a textbox. For example, if you type the letter ‘a’, all accent variants of the letter ‘a’ in that language will show up like this:
I think having hotkeys like this would save time when typing, and it removes the clutter of having ever accent option for every letter on the bottom of the screen.
This isn’t what you’re asking for but just in case you’re not aware of it… in Web you can hold down the Alt key as you press the letter concerned and it’ll cycle through each accent applicable to that letter in the current language. (In other words, while holding down Alt, if I press E once I’ll get the type of E in caffè, then if I press E again I’ll get the type in perché.)
I find it to be a pretty fast method of getting the accent in, though in Italian and German there are relatively few to cycle through.