Clang: Use the tree instead of the list for Clang-Tidy settings

List of checks does not give enough flexibility to
select/unselect specific checks. The tree fixes that.

Also remove Clang-Tidy checks line edit because it is
now integrated into the tree mode as an alternative way
of providing checks by pressing "Plain text edit" button.

'cpptools_clangtidychecks.h' is generated using python
script 'generateClangTidyChecks.py' and clang-tidy
from our LLVM/Clang 6.0 build.

Change-Id: I2ed1738cb2cbbf8dac6aba563469f06f69b11593
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/cpptools/cpptools.qbs

@@ -155,6 +155,7 @@ Project {
             "cppsourceprocessor.cpp",
             "cppsourceprocessor.h",
             "cpptools.qrc",
+            "cpptools_clangtidychecks.h",
             "cpptools_global.h",
             "cpptools_utils.h",
             "cpptoolsbridge.cpp",