Replace glyphspecs with the properties system

For now removed glyphspec handling code from Python and did preliminary changes to `renderer/glyph/*`.

- [x] `Glyph` vs. `BaseGlyph`. This is the major issue now. Those classes are distinct in the properties system but were merge together in JS.
- [x] Fix remaining issues after splitting `Glyph`.
- [x] ~~Update `common/plotting`.~~ EDIT: fix after this PR.
- [x] Update tests.