Improve searching code

Also simplified code a bit by removing unnecessary calls to toLower in TextPagePrivate::findTextInternalForward and TextPagePrivate::findTextInternalBackward I also fixed a small bug: the letter capital I with dot above (U+0130) did not match itself in case-insensitive mode on Qt 4.8.4 (U+0130 still does not match lowercase i (U+0069), which can be considered another bug, that I didn't fix (although this behavior conforms to the Unicode case folding rules)).

(I did not implement the Knuth-Morris-Pratt algorithm that I promised in a comment of Bug 323263 because on second thought I find that the win, if any, would probably be negligible except for some very special documents and special query strings.)

BUGS: 323262
BUGS: 323263
REVIEW: 112135