ClangFormat: Fix UI issues

- 'Default' values remain so after save/load
 - the language is fixed to C++
 - the project settings are shown correctly
 - fix parsing configuration file
 - do not trigger slots when we fill the table

Change-Id: I91b477721b5084803324cd38d0cfeb9d5650dd9f
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/clangformat/clangformatchecks.ui

@@ -4,7 +4,7 @@
  <widget class="QWidget" name="ClangFormat::ClangFormatChecksWidget">
   <property name="maximumSize">
    <size>
-    <width>450</width>
+    <width>480</width>
     <height>16777215</height>
    </size>
   </property>
@@ -2045,6 +2045,9 @@
      <property name="focusPolicy">
       <enum>Qt::StrongFocus</enum>
      </property>
+     <property name="enabled">
+      <bool>false</bool>
+     </property>
      <item>
       <property name="text">
        <string notr="true">Default</string>