Add SyncTeX support

This pull request should add SyncTeX support for the Skim opener, with keymap `ctrl-alt-s` (syncing on current line).

Probably these things need to be looked at before merging:
- [x] Instructions for the Skim side of configuration (preset in Skim requested: [Skim #1364](https://sourceforge.net/p/skim-app/feature-requests/1364/))
- [x] Does keeping track of the PDF file name work? Is it persistent across sessions? Is it per project/file or global as implemented?
- [x] Tests?