Simplify UseProjectSettings
Prevent NoSuchElementException in Extract trait.

Fixes #1001967
Fix race condition due to call of `Tree.symbol`

Calling `askOption` directly in the traverser is extremely expensive,
therefore it should happen as less as possible. Because the check if
the symbol exists is only necessary for dynamic types, which are not
used very often, it is only done when no other information in the tree
could be found.

Fixes #1002132
Merge pull request #711 from sschaef/t1002132-race-condition-due-to-symbol-call

Fix race condition due to call of `Tree.symbol`
Update UI after completion is applied and no linked mode is created

After the completion is applied to the underlying document, the editor
needs to be updated. Because the cursor position is independent from
the document content it needs to be updated separately, which was
forgotten in the case that no linked mode is created.

Fixes #1002151
Remove dead code in `CompletionProposal`
Merge pull request #717 from sschaef/t1002151-wrong-cursor-position-after-completion

T1002151 wrong cursor position after completion
Merge pull request #714 from huitseeker/issue/1001967

Prevent NoSuchElementException in Extract trait.
Simplifies AdditionalParametersWidget
Merge pull request #713 from huitseeker/issue/CustomCodeRemoval

Issue/custom code removal
Switch to using ScalaInstallation everywhere
Create preference page
lints the Scala Installation switch
Implements updateable, serialized Scala lib/compiler containers

Plugs in a list for Add Scala Library/Compiler
Fixes #1001777
Implements update of Scala Library Container based on SourceLevel