Clang: Restrict warning options edit to single line

...to avoid confusion.

Use a customized QPlainTextEdit instead of a QLineEdit because the
latter does not allow wrapping of long lines. Horizontal scrolling in a
QLineEdit for long/many options is inconvenient.

Change-Id: I15b5f034e04920d0ca955c4f822d882b05e9c235
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cpptools/cpptools.qbs

@@ -175,6 +175,8 @@ Project {
             "indexitem.h",
             "insertionpointlocator.cpp",
             "insertionpointlocator.h",
+            "wrappablelineedit.cpp",
+            "wrappablelineedit.h",
             "projectinfo.cpp",
             "projectinfo.h",
             "projectpart.cpp",