[matplotlib-devel] Tk backend different from others

I was working on my embedding chapter of my book when I noticed a subtle
difference in the tkagg backend from other interactive backends. First, all
other interactive Canvas classes inherit their respective backend's widget:

```
ben@tigger:~/Programs/matplotlib$ git grep "FigureCanvasBase" | grep "class
"
lib/matplotlib/backend_bases.py:class FigureCanvasBase(object):
lib/matplotlib/backends/backend_agg.py:class
FigureCanvasAgg(FigureCanvasBase):
lib/matplotlib/backends/backend_cairo.py:class FigureCanvasCairo
(FigureCanvasBase):
lib/matplotlib/backends/backend_gdk.py:class FigureCanvasGDK
(FigureCanvasBase):
lib/matplotlib/backends/backend_gtk.py:class FigureCanvasGTK
(gtk.DrawingArea, FigureCanvasBase):
lib/matplotlib/backends/backend_gtk3.py:class FigureCanvasGTK3
(Gtk.DrawingArea, FigureCanvasBase):
lib/matplotlib/backends/backend_macosx.py:class
FigureCanvasMac(_macosx.FigureCanvas, FigureCanvasBase):
lib/matplotlib/backends/backend_pdf.py:class
FigureCanvasPdf(FigureCanvasBase):
lib/matplotlib/backends/backend_pgf.py:class
FigureCanvasPgf(FigureCanvasBase):
lib/matplotlib/backends/backend_ps.py:class
FigureCanvasPS(FigureCanvasBase):
lib/matplotlib/backends/backend_qt5.py:class
FigureCanvasQT(QtWidgets.QWidget, FigureCanvasBase):
lib/matplotlib/backends/backend_svg.py:class
FigureCanvasSVG(FigureCanvasBase):
lib/matplotlib/backends/backend_template.py:class
FigureCanvasTemplate(FigureCanvasBase):
lib/matplotlib/backends/backend_wx.py:class
FigureCanvasWx(FigureCanvasBase, wx.Panel):
```
(I am going to assume that _macosx.FigureCanvas has all needed things
built-in, and I'll ignore the switched order for FigureCanvasWx). I have no
clue if this is potentially an issue or not, it just seemed a bit
inconsistent.

Second, while I haven't tried out all the backends yet, I noticed that the
Figure window for tkagg has an annoying border that the other backends
don't have. It is fairly wide, 4 pixels. I would like to get rid of that.
Does anybody object to that? I can make a PR for that and any other border
widths I find.

Cheers!
Ben Root