Refactorings for quickfixes

The relevance of quickfixes for the completion proposal is moved
to its own location in order to keep a better overview over them.
Exposed "Restart Presentation Compiler" action

Users can now restart the presentation compiler at will.  This can be handy
when false errors are reported in a compilation unit, and you want to reset a
Presentation Compiler without performing a full clean/build.

To manually restart a Presentation Compiler, a user has to right click on the
project's folder (in the Package Explorer), and select ``Scala > Restart
Presentation Compiler``.

In order to place the functionality inside the Scala menu (as described above),
I was forced to use an `action` (and not a `command`). I did try to use a
`command`, but I couldn't find the incantantion for placing the "Restart
Presentation Compiler" inside the Scala menu when using a `menuContribution`.

Fixes #1000555
Merge pull request #468 from dotta/experimental/reset-pc

Exposed "Restart Presentation Compiler" action
Merge pull request #469 from sschaef/quickfix-refactorings

Refactorings for quickfixes
Refactorings in quick fix + refactoring packages

This contains:

- Deletion of AddImportAction because it is unused
- Clean up imports of all components in the packages
- Small stylistic changes
- Additional Scaladoc
Resolves a bug that leads to no quick assistants

This was an interesting bug to fix. What happened here was that we
had a parallel collection that swallowed an exception. Because the
exception was never shown to the outside world it was not possible
to find out what went wrong. I managed to get the exception by
removing the parallel collection, which was done not because I knew
what to do but because the parallel collection looked unnecessary.

The exception itself was due to an unescaped string that is interpreted
as a regex and could easily be fixed. It occurred whenever one selected
a method with a symbolic identifier like '+'.

More interestingly, even after the fix of the bug the code with the
parallel collection still didn't work - it just returned an empty list.
I have no idea what went wrong but I believe its a problem somewhere in
the internals of the parallel collection - thus no usege of it anymore.

Fixes #1001801, #1001798
Merge pull request #477 from sschaef/t-quickfix-fix

T quickfix fix
Refactorings in ScalaSourceViewerConfiguration

The order of parameters in the constructors of the scanner classes
has changed.

All other changes are formatting related.
Merge pull request #432 from sschaef/t-refactor-source-viewer

Refactorings in ScalaSourceViewerConfiguration
Delete old plugin signatures
Merge pull request #478 from sschaef/t-delete-old-signatures

Delete old plugin signatures
Removes reference to non-existing command

Fixes #1001807
Merge pull request #480 from skyluc/issue/no-command-for-restart-pc-1001807

Removes reference to non-existing command
Sync 'isDefined' in presentation compiler cache.

This fixes one race condition when hyperlinking between
projects. Project 'client' depends on project 'base'.
The sequence of events was:

- dependent project 'base' starts building
- the indexer for `client` kicks in and starts the PC. The PC is not
  yet initialized. PC starts without `base` classfiles on its class path
- `base` finished building and triggers a reset of the PC for `client`,
  that normally restarts with `base` classfiles on the class path
- reset is ignored (PC is still starting, and `isDefined` returns false)
- hyperlinking fails, because the target is not available on the class path
Resolve exception when using type mismatch quick fix

This is a regression due to refactorings in 8f42b9de31c9365d6f9c.

The regex that was used to check an internal message was updated to
work with Scalas new string interpolation feature. Unfortunately,
while the regex was the same the semantics changed because it was
tried to insert and extract a value into and from the string
interpolator at the same time, which is in this situation not possible.

Fixes #1001809
Merge pull request #483 from dragos/fix/race-condition-cached

Sync 'isDefined' in presentation compiler cache.
Updated sbt.version to 0.13.0-RC4
Merge pull request #484 from dotta/transient/update-sbt-to-rc4

Updated sbt.version to 0.13.0-RC4
Merge pull request #479 from sschaef/t-fix-type-mismatch-quickfix

Resolve exception when using type mismatch quick fix