Fix auto-pairing brackets

Fixes #1574

This commit refactors key bindings again, to...

1. remove unnecessary `"operator": "equal"` and `"operand": true`
2. ensures common default order of "context" queries.
3. merge Default.sublime-keymap into platform specific files,
   because former one is not exposed by "edit_settings" command.
4. adds bindings to properly auto-pair brackets in math blocks.

Author: deathaxe