Add separate drawstyles options to Qt figureoptions dialog

[mfitzp]

This fixes #4447 where it was reported that it is impossible to
undo a step drawstyle using the Qt figureoptions dialog. The step
draw style was applied as a linestyle (which is permitted in the
API) but undo 'default' option was not included in the list.

However adding the 'default' to the linestyles would mean enabling
step and changing the linstyle would require the dialog to be
reopened. Here instead a separate option set is created for
draw style (and Style is renamed to Line Style).

There is also a change of indentation for the line:

 has_curve = bool(curves)

which was not indented into the correct block. This
caused an error on any figures without visible lines.

[jenshnielsen]

👍 Works as expected and looks like a clean solution to me

[WeatherGod]

what's the reason for the indent change here?

[tacaswell]

Because with out this and with no lines you get

 Traceback (most recent call last):
  File "/home/tcaswell/other_source/matplotlib/lib/matplotlib/backends/backend_qt5.py", line 653, in edit_parameters
    figureoptions.figure_edit(axes, self)
  File "/home/tcaswell/other_source/matplotlib/lib/matplotlib/backends/qt_editor/figureoptions.py", line 95, in figure_edit
    has_curve = bool(curves)
UnboundLocalError: local variable 'curves' referenced before assignment

If you suspect this is an IPython bug, please report it at:
    https://github.com/ipython/ipython/issues
or send an email to the mailing list at ipython-dev@scipy.org

You can print a more detailed traceback right now with "%tb", or use "%debug"
to interactively debug it.

Extra-detailed tracebacks for bug-reporting purposes can be enabled via:
    %config Application.verbose_crash=True

[WeatherGod]

By and large, this change makes sense. Drawstyles grew out of linestyles and it really should be separated now (it used to be a single concept). I am just concerned about an indent change.