Add setting for case-insensitive task matching

[Nuhser]

• Add a setting that turns case-insensitive task name matching on.
• If turned on, tasks with matching names ignoring the case will be treated as one.
• It needs to be decided if the grouping should either be handled during the summary creation or when the entry is created.
  • During summary creation: Would make it more flexible. Summaries would change when the setting is turned off and on again.
  • During entry creation: Would make the summary creation faster. Summaries wouldn't change anymore after the task is created independent of the current status of the setting.