First attempt on making a framework for extracting attributes from

First attempt on making a framework for extracting attributes from
selected text. This is in form of an AttributeExtractorExecutor that
works with AttributeExtractors. An AttributeExtractor is an ASTVisitor
that is also aware of the current text selection (a SmartTextSelection
that is a modified TextSelection made for easy access to the underlying
document).

This also includes an attempt on an AttributeExtractor for finding the
longest common prefix within a selection. The LongestCommonPrefixExtractor
is currently only working on plain text.

(The LongestCommonPrefixExtractor can be tested with the 'Test Attribute
Extractor' option in the 'IFI Refaktor Selection' editor context menu.)