Use Scala specific task markers.

Don't use Java task markers for Scala sources, as the Java builder
removes them all on "full build", which is what we trigger from the
Sbt build manager if:

* Java sources need recompilation
* resources (non-source files) exist on the source path (so they are 
  copied to the output folder)

This commit adds a Scala specific marker type (subtype of the Java one)
that preserves all functionality, but is only handled by our code.

Fixed #1001401, #1000975.
Organize imports.
Merge pull request #330 from dragos/isssue/tasks-disappear-1001401

Use Scala specific task markers.
Do not force default location of -XpluginsDir in Scala Compiler Preferences

Trying to force the default location to the plugins directory containing the
continuations.jar plugin turned out to be a very bad idea (this was done as
part of commit SHA-cbfd82d874f998b7726fec895ad4d83194ad1e5b).

The issue is that the default location where the continuations.jar is stored
changes *each time a user updates the Scala IDE* and, when updating, the
*former location is deleted*. The net effect of this is that after upgrading,
all projects using the continuations plugin would report the following error

	`bad option: -P:continuations:enable` is reported

Simply because the continuations.jar can no longer be located.

The fix is to somewhat restore the former implementation wrt how we handle the
`-Xpluginsdir` compiler setting. Basically, the solution consists in not
showing the default value of `-Xpluginsdir` to the user in the Preferences
dialog, and pass this default to the different compiler instance we create
unless the user specifices a different location (in which case, we do use the
location provided by the user).

There is one more catch due to the current way `ScalaProject.scalacArguments`
is implemented (the method is used to pass the compiler arguments to the Sbt
builder). In short, the returned arguments are only the ones forces by the user
via the Preference dialog. Since the `-Xpluginsdir` default directory is
**not** shown in the Preference dialog, the Sbt builder would fail to compiler
projects with continuations enabled because it wouldn't know where to look for
the continuations.jar. Hence, I had to add some ad-hoc code to handle this
correctly in `ScalaProject.scalacArguments`.

This whole fix is far from ideal, but I want to keep the changeset as small as
possible to avoid regressions, since this commit should be included in the
upcoming 3.0.0 release.

By the way, all existing projects that were affected by the error should
restore the defaults (Preferences > Scala > Compiler and hit the Restore
Defaults button).

Fixes #1001030, #1000700
Testing presentation compiler when project depends on continuations.jar

Added a couple of tests for making sure that the build compiler and the
presentation compiler behave the same when compiling projects that depend on
the continuations plugin.
Merge pull request #332 from dotta/issue/cannot-enable-continuations-plugin-1001030

Issue/cannot enable continuations plugin 1001030
Moved askReload calls to the Eclipse reconciler mechanism.

You can now register files for reloading in ScalaPresentationCompiler.scheduleReload
They are flushed at reconciliation, and reconciliation updates the error annotations

Fixing indentation.
proper structure for ErrorAnnotation update
De-activated test pending reimplementation of Eclipse model tests
Work on PR comments:better interfacing, plugging on IJavaReconcilingListeners
Essential tweaks in Problem annotation safety
Narrowed down invalidated text region