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
better scala container addition tagging
Basic status handler for the Xsource flag
Switch ScalaPresentationCompilerProxy.failedCompilerInitialization to Status Handlers
Centralizing thread Management for handlers
Aggregate Initialization Failures in ScalaCompiler reference validation
Switch UIErrorReporter to status handlers