A set of fixes for improving experience of file dialogs, mostly on Windows + some fixes for handling urls

KFileWidget:
- check whether a string contains protocol part more carefully because c:/ is an absolute path, not a protocol (static containsProtocolSection()), so we assume the protocol has at least two characters before ":/"
- enclose locationEdit->currentText() with QDir::fromNativeSeparators() and add .trimmed() to cover more cases in conditions
- slotOk(): add code for handling accepting using OK button when directory path is entered and directories are not acceptable result of the file widget:
cd to that directory then; in particuar it is now possible to enter C:\ or /
and press OK to change to the root dir on windows (or linux, respectively)

KUrlCompletion:
- construct the path more carefully because cwd can already end with '/', e.g. it can be "c:/"

KUrl:
- KUrl::equals() and ::urlcmp(): make comparison of local urls case-insensitive on Windows
- KUrl::hasSubUrl() some more optimizations