Diagnostics and Quick fixes for lost config elements

[dshimo]

Addresses #67

CodeActionParticipant, AddFeature:

• Add an initial draft for 'AddFeature' quickfix.
• The idea for the quickfix is to provide a list of direct features because transitive relations offer too many.
• As an enhancement, this quick fix could alternatively provide a list of some of the most commonly used features.

DiagnosticParticipant:

• Adds additional check while parsing the server.xml to recognize config elements.
• After the document is parsed, create relevant diagnostics for lost config elements.
• Add a trigger line for grabbing the feature list even if a FeatureManager is empty or not found

New FeatureListGraph class:

• Designed as a manager class for retrieving a desired FeatureListNode
• On assumption that all features and config elements have a unique identifier, this class uses a hashmap to provide constant time lookup to a node.
• To provide a list of transitive relations for features -> config element or vice versa, FeatureListGraph implements a breadth-first-search iterative strategy on a given node.

New FeatureListNode class:

• A new class to represent a generic node in a Feature List XML. It's intended to store features or config elements.
• isConfigElement operates on the heuristic that features provide major versions and that . isn't typical in a normal element name.
• 'Enables' refers to when this node automatically pulls in another feature or config element.
• 'Enablers' refers to features that would automatically pull in this node.

FeatureService:

• The priority for using a generated local feature list is placed higher. This xml file not only provides the list of features but also the necessary metadata for recognizing config elements and their relations.
• Build the FeatureListGraph while reading the feature list

Tests:

• Add check that diagnostics appear on expected locations
• Add check that the feature list xml gets digested within expectations

[cherylking]

My comment about the FeatureListGraph in FeatureService is significant and requires some code changes. Hopefully it's doable.

[cherylking]

See one comment about LOGGER variable.