Extract JetTestUtils.getLastCommentInFile()

JetNameSuggesterTest contains useful code for getting the last comment
of a file. Extract it to JetTestUtils.getLastCommentInFile() so other
tests can use it as well.
Make JetTestUtils.getLastCommentInFile() to allow trailing newline

Many editors add a trailing newline when saving. Allow them in
getLastCommentInFile.
Separate GUI logic from smartSelectExpression

Previously smartSelectExpression did both the determination of suitable
expressions and displaying the GUI. Separate the actual logic into a new
method, getSmartSelectSuggestions so that smart selection can be tested
in the future.
Add test suite for smart expression selector

Add test suite for smart expression selector (the small expression list
popup in e.g. Extract Variable.)

The test file format is similar to that in JetNameSuggesterTest: a .kt
file with the usual <caret> marker specifying the place where the smart
selector will be run. Then, the last comment in the file should contain
the expected outcome.
Fix unnecessary parenthesis around 'this' in refactorings

Operator priority calculation for 'this' expressions was incorrect in
JetPsiUtil.getPriority(): it would return a very low value for
a JetThisExpression, causing the IDE to report that parenthesis
are necessary in an expression like '(this)[1]'.

Fix this by making JetThisExpression not implement
JetStatementExpression by removing the implements clause from
JetLabelQualifiedInstanceExpression and adding it to all other
subclasses of JetLabelQualifiedInstanceExpression.

Fixes KT-4513.
Prevent smartSelectExpression from breaking up 'this@outerClass'

The previous commit made JetThisExpression not implement
JetStatementExpression. This had the side effect that
smartSelectExpression over a label-qualified this expression
(e.g 'this@outerClass') would now also list a plain 'this'
as a potential expression. Restore the old behaviour by adding an
explicit check for JetThisExpression.