Cancel reindex requests instead of internally queueing them

With the queue, the first reindex request for a series of them was
always sent and left to be handled completely, whilst only the last of
any subsequent requests was maintained in order to skip unnecessary
intermediate ones. This way the core did not have to play catch up.

The unfortunate part was that the first reindex request always went
through, even if the core was very busy handling other requests and
could not handle that first request until a while later, after which
that first request might also have been superseded. As the request had
already been sent, however, there was no way to cancel it.

As the core now has request cancellation support, it is simpler to let
all reindex requests pass through to the core and just cancel the ones
that aren't relevant. This automatically allows us to cancel even the
first request, at least if it wasn't being processed yet - but that was
also the case before.

In short, this should improve latency on slower hardware where many
actions are being performed simultaneously.

References #347

lib/IndexingMediator.coffee

@@ -1,5 +1,7 @@
 Popover = require './Widgets/Popover'
 
+CancellablePromise = require './CancellablePromise'
+
 module.exports =
 
 ##*
@@ -37,7 +39,14 @@ class IndexingMediator
      * @return {Promise}
     ###
     reindex: (path, source, excludedPaths, fileExtensionsToIndex) ->
-        return new Promise (resolve, reject) =>
+        reindexCancellablePromise = null
+
+        cancelHandler = () =>
+            return if not reindexCancellablePromise?
+
+            reindexCancellablePromise.cancel()
+
+        executor = (resolve, reject) =>
             @indexingEventEmitter.emit('php-integrator-base:indexing-started', {
                 path : path
             })
@@ -69,13 +78,17 @@ class IndexingMediator
                         percentage : progress
                     })
 
-            return @proxy.reindex(
+            reindexCancellablePromise = @proxy.reindex(
                 path,
                 source,
                 progressStreamCallback,
                 excludedPaths,
                 fileExtensionsToIndex
-            ).then(successHandler, failureHandler)
+            )
+
+            return reindexCancellablePromise.then(successHandler, failureHandler)
+
+        return new CancellablePromise(executor, cancelHandler)
 
     ###*
      * Initializes the project.

lib/ProjectManager.coffee

@@ -306,7 +306,7 @@ class ProjectManager
      * @param {String}      fileName The file to index.
      * @param {String|null} source   The source code of the file to index.
      *
-     * @return {Promise}
+     * @return {CancellablePromise}
     ###
     performFileIndex: (project, fileName, source = null) ->
         return @indexingMediator.reindex(
@@ -328,34 +328,15 @@ class ProjectManager
     attemptFileIndex: (project, fileName, source = null) ->
         return null if @isProjectIndexing()
 
-        if fileName not of @indexMap
-            @indexMap[fileName] = {
-                isBeingIndexed  : true
-                nextIndexSource : null
-            }
-
-        else if @indexMap[fileName].isBeingIndexed
-            # This file is already being indexed, so keep track of the most recent changes so we can index any changes
-            # after the current indexing process finishes.
-            @indexMap[fileName].nextIndexSource = source
-            return null
-
-        @indexMap[fileName].isBeingIndexed = true
-
-        handler = () =>
-            @indexMap[fileName].isBeingIndexed = false
+        if fileName of @indexMap
+            @indexMap[fileName].cancel()
 
-            if @indexMap[fileName].nextIndexSource?
-                nextIndexSource = @indexMap[fileName].nextIndexSource
+        @indexMap[fileName] = @performFileIndex(project, fileName, source)
 
-                @indexMap[fileName].nextIndexSource = null
-
-                @attemptFileIndex(project, fileName, nextIndexSource)
-
-        successHandler = handler
-        failureHandler = handler
+        onIndexFinish = () =>
+            delete @indexMap[fileName]
 
-        return @performFileIndex(project, fileName, source).then(successHandler, failureHandler)
+        return @indexMap[fileName].then(onIndexFinish, onIndexFinish)
 
     ###*
      * Indexes the current project asynchronously.

lib/Proxy.coffee

@@ -1037,7 +1037,7 @@ class Proxy
      * @param {Array}         excludedPaths          A list of paths to exclude from indexing.
      * @param {Array}         fileExtensionsToIndex  A list of file extensions (without leading dot) to index.
      *
-     * @return {Promise}
+     * @return {CancellablePromise}
     ###
     reindex: (path, source, progressStreamCallback, excludedPaths, fileExtensionsToIndex) ->
         if typeof path == "string"