Material-ui: [DatePicker] Provide prop to customize the icons

[Philipp91]

The icons are currently hard-coded:

https://github.com/mui-org/material-ui/blob/29fbcceb44e851a3a856e42035f86865dacd1fcd/packages/material-ui-lab/src/internal/pickers/PickersToolbar.tsx#L98-L102

There is already a keyboardIcon prop, but that one controls the icon in the input adornment. I don't really understand why that prop has "keyboard" in its name. It does not show a keyboard on the icon. And one would click that button precisely when _not_ using the keyboard but rather the mouse to input a date. I'd call that one openIcon (by function) or calendarIcon (by icon content).

[dmtrKovalenko]

Yeah, it is reaaly legacy name. Should be nice to change in scope of v4-alpha breaking changes

[dmtrKovalenko]

Would you like to open PR?

[Philipp91]

Well it's not just about renaming the existing prop, but also about exposing new props for those two hard-coded icons.

[dmtrKovalenko]

Yeah so I propose these changes:

1. Rename keyboardIcon => calendarIcon
2. Use this icon also for rendering mobile keyboard input view
3. Add new prop editIcon for the mobile keyboard input view

[Philipp91]

Sure, I can take a stab at this. I can't assign myself in GitHub, but stay tuned.

[Philipp91]

Yeah so I propose these changes:

1. Rename keyboardIcon => calendarIcon

Naming the prop after the icon content doesn't play well with this usage, where we'd end up with calendarIcon: <ClockIcon/> even those two are sort-of opposites and shouldn't be equated like that.

So perhaps openIcon or something else in the direction of the icon's functionality (adornmentIcon, buttonIcon) would be better?

[dmtrKovalenko]

Hmm really, openPickerIcon should be nice?

[Philipp91]

1. Use this icon also for rendering mobile keyboard input view
2. Add new prop editIcon for the mobile keyboard input view

Just like for the other one, I don't think we should name the props after the icon content. How about mobileKeyboardViewIcon (that's the pen that switches to the textbox/keyboard view) and mobileDefaultViewIcon (that's the calendar icon that switches back to the regular mobile view)?

[oliviertassinari]

Regarding the resolution, I would suggest we go in the direction of #21453. Meaning a components prop with keys for each customization points.