Three delimited text bug fixes, GUI tidyup, context help fix

This pull request includes the following.

1) Bug fix - User settings not properly restored when dialog is opened or new file loaded. This is a bug I introduced (missing ! character), and I believe must be fixed.

2) Bug #6719 - Was checking through bug log for bugs related to delimited text plugin in and noted bug #6719 regarding to comma as decimal point option in the delimited text plugin.  This patch fixes the behaviour so that the decimal point option  applies  to all potentially numeric columns.  Includes minor UI fix moving decimal point to more logical place (ie not specifically associated with X,Y column), code fixes, and new test case for decimal point option.

3) Bug  fix - the  context help files had not been renamed since the GUI component got moved to QgsDelimitedTextSourceSelect, so context help was not working.  Context help files have been renamed.

4) Tidy up of UI to ensure logical and consistent names for items (eg not referring to fields as columns), added visual grouping of file format options and geometry definition options, to hopefully improve clarity for user. 

5) Update of context help file (en_US version only - sorry) to more correctly describe usage of delimited text.