Consistent search settings between document search and global search

[jeremypw]

Fixes #1277

For simplicity the global search dialog can no longer change search settings - this is only done in the searchbar. The global search dialog instead just informs the user of the current settings.

If required these settings could also be changed in the global search dialog and synchronised to the search bar.

Some other minor fixes/refinements in passing:

• Validate regex expressions in the global search dialog (but not in the searchbar currently)
• Implement missing wholeword global searching
• Implement "Mixed case" setting in global searching
• Disable other settings when "Use Regex" is active.