More delimited text bug fixes

Additional tests cases, resultant bug fixes, documentation updates.  Main components of this pull request are:

Regular expression delimiter:  fixed non-intuitive behaviour of not
including capture groups in the returned fields, and improved behaviour
for anchored regular expressions.  Added test cases for handling of
regular expressions.

File encoding: Selected file encoding was being ignored.  Fixed and
added test cases for file encoding.

Bad formatted quoting: Fixed handling of badly formatted quotes (eg
unclosed quotes).  Added test case.

Field names: Where a file had duplicate or blank field names it was
not possible to reliably set the names of X,Y,WKT fields.  Relocated
field name handling to simpler common code in QgsDelimitedTextFile, with
automatic renaming of fields with ambiguous or missing names.  Add test
case for field renaming

User interface fixes: Fixed tooltip and other messages throughout.
Changed encoding combo box to be alphabetically sorted. Fixed data
sample grid to be refreshed when encoding changed.

Context help: Added documentation for using delimited text layer in python.