Update text color behavior for miscelleneous widgets and other widget improvements

Just randomly went around to find some widgets whose behavior could be easily updated.

First note - as time_picker is vendored, I don't know if I should dive into its code to change it (it's not a hard change, but would like feedback from @mixedCase first).

Second note - this also implements the accent picking I mentioned in https://gitlab.com/kivymd/KivyMD/issues/58 (without the automatic generation @ZingBallyhoo recommended, which I'm not fully convinced of the merits of). However in the process of doing this I've refactored the color selection buttons to their own class, which should make automatic generation slightly easier if anyone sees a need.