Excluded group setting for search.

This pull request is to allow users to specify group patterns that should be excluded from search results. Currently, 'Backup' is excluded by default, however depending on the plugin, localization or user preferences, the backup folder might be named differently (in my case, it was localized to French language). Also, some users might want to manually put aside certain passwords without deleting them completely. That might happen, for example, when certain passwords are required for a project but later becomes unneeded once the project is complete. You might want to keep them but don't need to show up in search results.

The proposed feature basically allows a bit more flexibility on what should be excluded. The following are the main changes:

- Added setting to allow users to exclude certain groups from search results (based on wildcard pattern).
- Added corresponding Search panel in Settings window.
- Added 'Backup' pattern by default, but users can remove it, as needed.
- Removed hard-coded exclusion of 'Backup' group, since the name might be different depending on plugin or localization settings.

I didn't add any tests yet but I don't mind doing so if needed.