GrepView: Extend default file extensions to search
Extends the default file extensions to search by *.txt,*.xml,*.rc as proposed in: https://bugs.kde.org/show_bug.cgi?id=402207
Also extends the header extensions by *.cuh (CUDA headers) and the C sources by *.cu (CUDA sources).
Test Plan: Tested it manually. Note: If one manually changed the file extensions in a session before, the new defaults won't show up there since they are overridden by the previously modified value.
Reviewers: KDevelop, arrowd
Reviewed By: arrowd
Differential Revision: https://phabricator.kde.org/D17892