Update plot_align_svg.py now that add_brain_schematics has been removed from displays

Tweak it to use additional arguments to ease alignment + remove commented code
Fix bug for display_mode != 'ortho'
Minor renaming/tidy-up
* BaseAxes.draw -> BaseAxes.draw_2d
* cut_ax -> display_ax
* other minor changes
Rename slicers.py -> displays.py
* Update doc to use display rather than slicer
* Update slicers imports