Initial file search implementation

See #1165.
Comment out wasteful DEBUG statements
Change "Search Books" to "Search books" for consistency with other search dialogs
fix strcoll() workaround

The strcoll() workaround we had in place for Kobo devices was (or has
become) ineffective. We had set self.strcoll to nil on Kobo devices -
but this was the instance variable. Setting it to nil effectively makes
the instance variable vanish, so when trying to access it later, it
was not there and got looked up via the metatable - which had the original
reference. Setting it to nil had no effect whatsoever.

We simplify that approach and set the replacement function where before we
had set this to nil.

This is a partial fix for issue #1183 (and explains a comment in issue #686
which says that the old fix did not work).

However, to really fix #1183 - if we want to do so - we would need a collate
function that normalizes uppercase/lowercase before compare.
implement case-insensitive sort, make default and configurable

This will convert any file name to lowercase before doing the comparison.
Note that this will only work for ASCII character range, a full Unicode
aware solution will be much more complicated. And in the end, file names
are byte arrays, not character strings ;-)

fixes #1183.
Merge pull request #1189 from hwhw/master

fix strcoll() workaround
Merge pull request #1186 from Frenzie/filesearcher

Initial file search implementation
(really?) fix refresh viewport calculations

This adds more documentation about the viewport offset calculations
for refreshing rotated viewports.

The old behaviour also was (still) buggy, even after the latest round
of fixes. Now, the offsets when rotated are hopefully calculated correct.
Merge pull request #1191 from hwhw/master

(really?) fix refresh viewport calculations
add support for GIF and PNG to picdocument provider

needs update to latest base
update to latest base
Merge pull request #1192 from hwhw/master

Add support for viewing PNG and GIF files
add fulltext search for PDF documents
fix #1121
Merge pull request #1194 from chrox/pdf_search

add fulltext search for PDF documents
Merge pull request #1195 from chrox/fix_1121

fix #1121
add an option to show an arrow to indicate page overlap
add missing translation of 'Screensaver' options
Merge pull request #1197 from chrox/fix_864

add an option to show an arrow to indicate page overlap
Tweak full refresh coutner logic

Don't increase it for regional updates