org-pdftools integration

[fuxialexander]

This PR integrates org-noter with org-pdftools. By using org-pdftools links as the new format to record the location, it's now possible to specify the exact annotation you want to link to.

See https://github.com/fuxialexander/org-pdftools for more detail.
DEMO: https://www.youtube.com/watch?v=lCc3UoQku-E

Most commands now work just fine, including a new command to convert old format notes to the new format: org-noter-conver-old-notes and a command to jump from pdf-annot to specific heading org-noter-jump-to-note.

• org-noter-create-skeleton now works fine

Things haven't finished yet:

• org-noter-create-skeleton
• A pdf annotation -> org importer function. Possibly called when session starts to ensure one-to-one sync between org and pdf annotations.
• A org->pdf annotation export function. Possibly called at session kill to ensure one-to-one sync between org and pdf annotations.

[weirdNox]

This was a very big change, so I had to take a careful look.
I ended up separating the org-pdftools specific stuff into the org-noter-integration.el, which interfaces with the package using hooks I created by looking at what you needed to do.
This way, the location references can be extended into another type, if anyone wants, without me interfering at all.
You should now improve org-noter-integration.el and ship it with your package or in a separate package.
Please take a look at the hooks I created, so if you need more of them you know what to ask :)
I think this is in feature parity with what you had, with the exception of the create skeleton, which is probably broken, it needs some hooks in there!