Update SummaryGenerator module; fix library dependencies (NuGet)

- Update SummaryGeneratorTest.
- Adjust SummaryGeneratorTest code so that it works with recent changes to
  SearchSummary module.
- Add SummaryGeneratorFactory.
- Update ISummaryGenerator interface.
- Add fileReader field to MatchingFile struct.
- Fix external library references.
- Add libraries to multiple projects via NuGet.
Add PDFDocumentBuilder class and their tests

Implement header and save metod.
Rewrite AddHeader method and add few others

Change library PDFSharp to MigraDoc.
Write internalTest for PDFDocumentBuilder.
Add congig file
Add search result context in reports - WIP

- Add extracting context & result positions to a more useful class
- Add position-aware stream reader
- Add IDocumentBuilder.AddText
- Add multiple tests for PositionTestPair and PositionTestPairGroup
- Adjust existing tests so that they pass with recent changes
Add automatic NuGet package downloading on build
Fix broken assembly references, insert placeholder PDFDocumentBuilder.AddText
Fix PDFDocumentBuilder.Save and its test
Add Begin/EndContextBlock, rename AddText to AddContextText in IDocumentBuilder
Add implementation to changed interface
Implement PDFDOcumentBuilder.

Imoplement methods from PDFDucumentBuilder. Fixe some bugs. Make changes
in PDFBuilder main test.
Add generating context blocks; add entry limit and context size to GUI

- Add calls to Begin/EndContextBlock in SummaryGenerator
- Add controls to toggle generating context in report, set context size and
  limit number of file entries in generated report
- Fix tests so hat they use new IDocumentBuilder interface
Add DocumentBuilderFactory + tests
Make SearchSummary use DocumentBuilderFactory

- Make the module actually use DocumentBuilderFactory
- Make DocumentBuilderFactory.Create case-proof
- Add DocumentBuilderFactory test case for valid format, invalid case
- Remove 'File name' checkbox from GUI (force it being written every time)
Add ellipsis between context groups in reports
Refactor SummaryGenerator class.

- Extract PositionTextPair, PositionTextPairGroup, PositionAwareStreamReader
  classes to separate files
- Add more text to SummaryGenerator tests
Make SummaryGeneratorFactory public; disable broken TxtDocumentBuilder.
Add "Export" button in the main form (plug in the SearchSummary module)
Merge remote-tracking branch 'origin/SearchSummary' into SearchSummary

Conflicts:
	FileScanner.SearchSummary/DocumentBuilderFactory.cs
	FileScanner.SearchSummary/SummaryGeneratorFactory.cs