Attempt to refactor SVGMobject with svgelements

[YishiMichael]

Motivation

Try using external library svgelements to refactor SVGMobject.

Proposed changes

• M manimlib/mobject/svg/svg_mobject.py: use svgelements to parse svg files
• M manimlib/mobject/svg/tex_mobject.py: remove override attribute from init_colors method; cut off global style-applying process from init_colors
• M manimlib/mobject/svg/text_mobject.py: remove init_colors inheritance; add set_fill process since svg files rendered by Text have the black color specified
• M manimlib/mobject/svg/mtex_mobject.py: replace .fill_color with .get_fill_color()
• M manimlib/mobject/mobject.py: remove override attribute from init_colors method
• M manimlib/mobject/geometry.py: add Polyline class; remove override attribute from init_colors method
• M manimlib/mobject/types/surface.py: remove override attribute from init_colors method
• M manimlib/mobject/types/vectorized_mobject.py: remove override attribute from init_colors method
• M requirements.txt: update requirements (add svgelements, remove cssselect2)
• M setup.cfg: update requirements (add svgelements, remove cssselect2)

Notes

• For each submobject of SVGMobject, 5 attributes (fill_color, fill_opacity, stroke_width, stroke_color, stroke_opacity) depend on the result of style parsing. If not specified, they will be set as default attributes passed to SVGMobject class. set_fill, set_stroke methods can still apply style globally.
• unpack_groups attribute is removed as the structure may be modified. Now SVGMobject always unpacks submobjects.
• Some changes may also have to be made to MarkupText, just like Text, but I cannot run MarkupText and cannot look into svg files rendered by it.
• <text> tags in svg are still not handled.
• A scaling process with scalar 0.75 is applied so that the size of a mobject remains as before.
• The outer svg tag may also contain style, which is ignored when inserting a group tag in svg content. This may be handled in the future.
• I notice that it's more reasonable to let SingleStringTex and MTex both inherit from SVGMobject, and in community edition they do. This may also be handled later.