addWatch: don't cancel other watches

[UnkindPartition]

Previously, addWatch would override any actions already registered with the
same watch descriptor (i.e. related to the same path). This commit changes the
behaviour so that the previously registered actions are kept.

Here's the test I used for this:

import Control.Concurrent
import Control.Exception
import Control.Applicative
import System.INotify

main = do
  ino <- initINotify
  let foo = "foo"
  writeFile foo "blah"

  _ <- addWatch ino [Open] foo $ \e -> putStrLn "opened"
  evaluate =<< length <$> readFile foo
  threadDelay (10^6)

  _ <- addWatch ino [Access] foo $ \e -> putStrLn "accessed"
  evaluate =<< length <$> readFile foo
  threadDelay (10^6)

It's in a different style than the project's tests (it registers the actions rather than just gathering the events), so I didn't include it, but feel free to adapt it for the test suite.

[UnkindPartition]

Good point, I didn't think of that. See the new commit.

[UnkindPartition]

(reminder)

[kolmodin]

Released 0.3.7 with your changes.