Feature Request

Feature summary
I'd like to be able to add the QuiverKey object to the legend as a handle. So the length of the arrow in the legend is meaningful to the quivers on the plot, but so it can be displayed in a tidy way with more mixed plot types. This wasmentioned on another issue, but I don't think it's actually been implemented.

Code for reproduction

frommatplotlib.patchesimportPatchimportmatplotlib.pyplotaspltimportnumpyasnpfig,ax=plt.subplots()X=np.arange(-10,10,1)Y=np.arange(-10,10,1)U,V=np.meshgrid(X,Y)q=ax.quiver(X,Y,U,V)qk=ax.quiverkey(q,0.9,0.8,U=10,label='QK length = 10',labelpos='E')legend_elements= [Patch(facecolor="red",edgecolor="black",label="First Element"),qk]ax.legend(handles=legend_elements)plt.show()

Actual outcome

/opt/conda/envs/py37/lib/python3.7/site-packages/ipykernel_launcher.py:20: UserWarning: Legend does not support <matplotlib.quiver.QuiverKey object at 0x7fcf101d7b00> instances.A proxy artist may be used instead.See: http://matplotlib.org/users/legend_guide.html#creating-artists-specifically-for-adding-to-the-legend-aka-proxy-artists

Expected outcome
Have the quiverkey as another element in the legend box with my other proxies. If I create a proxy as the error suggests, it will lose it's meaningful length and therefore it's value as a "key".

Matplotlib version

• Operating system: Linux Mint
• Matplotlib version: 3.1.2
• Matplotlib backend: module://ipykernel.pylab.backend_inline
• Python version: 3.7.3