mouse-based panning & zooming, SyncTeX support

[autc04]

Here are two things I hacked up because I wanted them for myself; I'm not sure how they fit with your idea of what atom-pdf-view's feature set should be.

• SyncTeX support - for TeX users. A click on a PDF file will open the corresponding place in the TeX source.
• panning & zooming with the mouse.

The two features interact slightly because the SyncTeX behaviour is suppressed when the mouse moved during the click; they can easily be separated if you want to include one but not the other.

[izuzak]

Hey, @autc04! Thanks for opening a pull request :green_heart:

I'd be interested in adding these features. Before reviewing, could I trouble you to split this pull request into two smaller pull requests, one for each feature? That will make reviewing, testing and merging a bit easier since the features are mostly independent.

You mentioned that the features do interact a bit, so perhaps we could start with a pull request for "panning & zooming with the mouse.", and then open a pull request for SyncTeX support once that is in? :thought_balloon:

[autc04]

done. autc04-mousepan can now be reviewed first.

[autc04]

I'll add some more polish and reopen soon.