Changes

[321775] Allow parsing Dot Html-Like-Label comments in Html tags.

- Modify the DotHtmlLabel grammar to send the HTML_COMMENT elements
nested in a HtmlTag to the hidden channel.
- Implement corresponding DotHtmlLabelTests parser/validation test
cases.

[321775] Improve Dot Html-Label attribute value validation support.

- Extend the DotHtmlLabelJavaValidator by attribute value validation
rules.
- Implement corresponding validation test cases.

[321775] Improve Dot Html-Like-Label folding support.

- Provide a custom DotFoldingRegionProvider where the folding regions
for the dot HTML-Like-Label attribute value are computed as well.
- Modify the Dot HTML-Like_Label folding not to calculate the folding
regions for html content elements containing only whitespaces.
- Implement corresponding DotFoldingTests test cases.

[321775] Improve Dot Html-Like-Label outline view support.

- Extend the GenerateDot.mwe2 workflow to generate labeling/outline API
for the DotHtmlLabel.xtext sub-grammar.
- Provide implementation for the DotHtmlLabelLabelProvider and
DotHtmlLabelOutlineTreeProvider classes.
- Provide html_tag.png and html_text.png icons used by the outline view.
- Modify the DotLabelProvider and the DotOutlineTreeProvider to consider
the DotHtmlLabelLabelProvider and the DotthmlLabeloutlineTreeProvider
implementations when creating the outline view.
- Implement corresponding DotOutlineViewTests test cases.

[461506] Improve the Dot Color sub-grammar validation/content assistant.

- Modify the DotColorJavaValidator and the DotColorProposalProvider to
consider the globally set colorscheme attribute value.
- Modify the DotColorJavaValidator and the DotColorProposalProvider to
treat the colorscheme attribute value case insensitively.
- Move the Dot ast walking logic from the DotProposalProvider class into
the DotAstHelper class.
- Implement corresponding validation and content assistant test cases.

[461506] Add colorList Dot Attribute type (bgcolor, color, fillcolor).

- Define colorList sub-grammar: DotColorList.xtext.
- Change the Dot Attribute type from color to colorList where necessary:
	bgcolor   G colorList
	bgcolor   C colorList
	color     E colorList
	color     C color
	color     N color
	fillcolor E color
	fillcolor N colorList
	fillcolor C colorList
- Adapt/Implement DoAttributesTests, DotContentAssistTests,
DotParserTests and DotValidatorTests test cases.

[461506] Edge fillcolor DOT attribute proposal provider improvement.

- The edge fillcolor attribute accepts only a color (and not a
colorList) as a valid value. Adapt the DOT Editor proposal provider
to offer only color value. So this behaviour is consistent to the DOT
attribute validator.
- Improve DotAttributes documentation.
- Adapt the corresponding DotContentAssistTests test cases.

[NONE] dot.tests.DotTestGraphs: improve copyright and documentation.

[520598] Use extension point for DOT sub-grammar package registration.

- Merge the org.eclipse.gef.dot/plugin.xml_gen (generated by the Xtext
framework) into the org.eclipse.gef.dot/plugin.xml file to properly
register the generated DOT sub-grammar Ecore packages via the
"org.eclipse.emf.ecore.generated_package" extension point. This makes
the Dot test cases more stable and robust.
- Eliminate the programmatic EPackage registrations from the
DotValidatorTests.java file, since this is not necessary any more.

[518893] Prevent processing of non-viewer-related events.

 - Prevent TypeStrokeGesture from passing a null IViewer to
   the IHandlerResolver.
 - The IViewer for an event can only be null, if the event was
   fired for a JavaFX node that is not part of the viewer.