Building on #1874, commit adds user controls to choose
or edit style in cppcheck-gui ONLY. Commit does not
address CodeEditor style usage in triage app at this time.
Code Editor style can be altered from the added "Code Editor"
tab in the user preferences. The user has the option to select
default light, default dark, or to customize.
If user leaves the style set to light or dark defaults, this
will be reflected in the choices shown in the preferences
dialog.
User choice for Code Editor Style is saved in the cppcheck-gui
preferences under the heading "EditorStyle".
