Better support for auto-completion of identifiers that require backticks

Now, identifiers (of classes, traits, defs, fields, packages (and more?)) that would be invalid without back-ticks (including identifiers that are reserved words in scala, say, a method named 'yield' in some java lib) are auto-completed with back-ticks.

This PR is related to ticket [#1001371](https://www.assembla.com/spaces/scala-ide/tickets/1001371#/activity/ticket:). The ticket demands 2 enhancements:
* Auto-completion should add back-ticks for identifiers that would require them.
* Auto-completion suggestions should be made available by the IDE, when the user starts writing the term (to be auto-completed) both with & without a leading back-tick.

This PR does not address the 2nd form of the 2nd enhancement, i.e. auto-completion when the user has already typed a leading back-tick is not supported.