Added autocomplete for constructors.

This was done by adding an additional case to the completion logic
in ScalaCompletions.

To avoid problems with completion overwrites, similar to Eclipse Java
only tooltips are shown for constructor completion.

A known defect is the ability to complete constructors of classes with
mixins on the same line. See #1001950

Fix #1001272
Remove rename proposal from quick assists.

Checking for initial conditions requires a full project index, leading
to unnecessary long wait times. As the name implies, we'd like to keep it fast,
especially since it happens on the UI thread.

Fixed #1001947.
Merge pull request #581 from dragos/issue/slow-quick-assist-1001947

Remove rename proposal from quick assists.
Merge pull request #580 from danchia/add-autocompletion-for-new-t1001272

adds constructor autocomplete. t1001272
Added note about PR getting close after 2 weeks of inactivity
Merge pull request #583 from dotta/doc/update-contributing-notice

Added note about PR getting close after 2 weeks of inactivity