Updated the appearance of Slider widgets

The appearance of `~.Slider` and `~.RangeSlider` widgets

were updated and given new styling parameters for the

added handles.

.. plot::

import matplotlib.pyplot as plt

from matplotlib.widgets import Slider

plt.figure(figsize=(4, 2))

ax_old = plt.axes([0.2, 0.65, 0.65, 0.1])

ax_new = plt.axes([0.2, 0.25, 0.65, 0.1])

Slider(ax_new, "New", 0, 1)

ax = ax_old

valmin = 0

valinit = 0.5

ax.set_xlim([0, 1])

ax_old.axvspan(valmin, valinit, 0, 1)

ax.axvline(valinit, 0, 1, color="r", lw=1)

ax.set_xticks([])

ax.set_yticks([])

ax.text(

-0.02,

0.5,

"Old",

transform=ax.transAxes,

verticalalignment="center",

horizontalalignment="right",

)

ax.text(

1.02,

0.5,

"0.5",

transform=ax.transAxes,

verticalalignment="center",

horizontalalignment="left",

)