Migrate colors and icons variables to Enums

[ndonkoHenri]

Description

Fixes #4169

Test Code

import flet as ft

print(ft.colors.BLACK)
print(ft.colors.RED.with_opacity(0.5))
print(ft.colors.random())

print(ft.icons.WB_SUNNY)
print(ft.icons.random())


# deprecated - use random() instead
print(ft.icons.random_icon())
print(ft.colors.random_color())


# broken
print(ft.colors.with_opacity(0.5, "blue"))

Note

• I named the enums colors and icons (lowercased) mainly for backward compatibility, however the python convention is to have class names in CapWords. We could actually have both then deprecate the lowercase.
• Once we conclude with the right way to do this, I will proceed with updating the cupertino icons/colors then the controls with color-props.

[FeodorFitsner]

I'm not sure about MaterialIcons and MaterialColors. Material is the default theme in Flet. I like just Colors and Icons more.

[FeodorFitsner]

...and cupertino is rarely used, so let's leave Colors and Icons.