option to strip requested prefix index in the list

[[Special:PrefixIndex]] could be transcluded to generate a dynamic list
of pages. Since all pages share the same prefix, it is not always
wanted, specially when looking for subpages.

Given articles named:

- Project/Alix
- Project/FixBug20281
- Project/S3cretPlan
- Projects roadmap

{{Special:PrefixIndex/Project}} yields:

  Project/Alix   Project/FixBug20281  Project/S3cretPlan
  Projects roadmap

Adding |stripprefix=1 get rid of the prefix:

{{Special:PrefixIndex/Project|stripprefix=1}}

   /Alix      /FixBug20281     /S3cretPlan
   s roadmap

{{Special:PrefixIndex/Project/|stripprefix=1}}

   Alix      FixBug20281     S3cretPlan

The patch adds a checkbox to activate the prefix stripping.

For the implementation, I have chosen a protected class property instead
of adding parameters to various methods.  That seems easier to handle.

Change-Id: I96a0a08ea11a82016eb382abbeb2d54bfc2c4016