Improve code coverage for GUI classes

[pbludov]

Current coverage:

Class	Coverage
BaseCellEditor	8%
TreeTable.TreeTableCellEditor	8%
TreeTableCellRenderer	35%
TreeTable	89%
ListToTreeSelectionModelWrapper	63%
ParseTreeTableModel	80%
TreeTableModelAdapter	56%
CodeSelector	80%
TreeTableModelAdapter.UpdatingTreeModelListener	60%

The goal is 100% code coverage.

Notes

It is possible to run GUI from IDE and collect the coverage report:

Some code may be unreachable from any user input (mouse, keyboard, changing system properties and so on).
Such code should be considered as "dead" code and should be deleted.

Before deleting any code, make sure that it is not executed in Windows, Mac & Linux.

Please send one PR per class. Use existing GUI tests from the package com.puppycrawl.tools.checkstyle.gui for reference.

remove exclude:

checkstyle/pom.xml

Lines 834 to 849 in 70946a8

	<!-- Swing related classes -->
	<exclude>com.puppycrawl.tools.checkstyle.gui.BaseCellEditor</exclude>
	<exclude>com.puppycrawl.tools.checkstyle.gui.CodeSelector</exclude>
	<exclude>
	com.puppycrawl.tools.checkstyle.gui.ListToTreeSelectionModelWrapper
	</exclude>
	<exclude>com.puppycrawl.tools.checkstyle.gui.ParseTreeTableModel</exclude>
	<exclude>com.puppycrawl.tools.checkstyle.gui.TreeTable</exclude>
	<exclude>
	com.puppycrawl.tools.checkstyle.gui.TreeTable.TreeTableCellEditor
	</exclude>
	<exclude>com.puppycrawl.tools.checkstyle.gui.TreeTableCellRenderer</exclude>
	<exclude>com.puppycrawl.tools.checkstyle.gui.TreeTableModelAdapter</exclude>
	<exclude>
	com.puppycrawl.tools.checkstyle.gui.TreeTableModelAdapter.UpdatingTreeModelListener
	</exclude>

and all further more specific low level coverage, like:

checkstyle/pom.xml

Lines 868 to 882 in 70946a8

	<rule>
	<element>CLASS</element>
	<includes>
	<include>com.puppycrawl.tools.checkstyle.gui.BaseCellEditor</include>
	</includes>
	<limits>
	<limit>
	<counter>LINE</counter>
	<value>COVEREDRATIO</value>
	<minimum>0.09</minimum>
	</limit>
	<limit>
	<counter>BRANCH</counter>
	<value>COVEREDRATIO</value>
	<minimum>0.00</minimum>

[romani]

@pbludov, we need to enforce current coverage number, to make sure CI will not allow going down.
Human can not track this.

[romani]

One more improvement is merged

[romani]

We do not do assignments, just make a comment "I am on it" and start a work. We appreciate your help.