Update to new quickAssist extension point of scala-ide core

Needs to be merged after scala-ide/scala-ide#819

- Remove `javaResolver` because it seems to be unnecessary
- Remove behavior of `hasAssist` method because it is duplicated with
  the `compute` (former `getAssists`) method
- Replace class `BaseAssistProposal` with `RouteEntryProposal`

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I don't understand the purpose of `javaResolver`. Is this one really unneeded?