CreateMethodProposal refactoring

To reuse behaviour in abstract members implementation feature
Abstract members implementation added

Current version limitations:
1) There is no stub generation for abstract val/vars
2) Type parameters for defs are not generated
3) The text overlaps with the icon in quick fix menu when there is one abstract method
4) Additional brace is added to the class while invoking abstract member implementation feature from class/trait with empty body:
class Test extends TestTrait {^
}
(but for: class Test extends TestTrait {^} result is correct)
5) Stub for abstract member is not generated while invoking abstract member implementation feature if the cursor is on `class`/`trait` keyword: cla^ss Test extends TestTrait
Stubs for abstract members - type params, val/var added
Stubs for abstract members - tests added
Merge pull request #672 from VladimirNik/abstract-members-implementation

Stubs for abstract members
Now, identifiers (of classes, traits, defs, fields, ...) that would be
invalid without back-ticks are autocompleted with back-ticks.

Also added tests that exercise this enhancement.
Merge pull request #688 from pmellati/literal-identifiers-autocomplete-1001371

Better support for auto-completion of identifiers that require backticks
Attempt to not crash too badly when a file is deleted from the workspace.
Introduce ScalaInstallation to represent the Scala jars used to build a project.
Merge pull request #691 from dragos/issue/on-the-fly-compiler-interface

Define ScalaInstallation, in preparation for multi-version support
Bumps version numbers
Increase test timeouts (possible source of flaky classpth tests).
Create features for each Scala versions

This will allow to install multiple versions of Scala on
the same Eclipse installation.
The main sdt build phase now only depends on p2 repos, external or local
created in the previous phases.
Some version definition
Tweaks the arguments in the build script
Creates a Scala 210 jars bundle

To be used instead of the normal bundles until when are able to support multiple bundles of Scala
Fixes scala211 source feature label
Adds computeSourcePath documentation
Cleans up new features labels
Merge pull request #690 from skyluc/issue/multi-scala-support

multi Scala support