GrepView: Extend default file extensions to search

Summary:
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).

FEATURE: 402207

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

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D17892