Draft: Tweak nautilus-search-popover design a bit

Currently, some labels from nautilus-search-popover aren't very clear:

• "What": doesn't explain what it does
• "Search": doesn't fully explain what it does
• "When": looks a bit vague

This commit makes the following changes to these labels:

• What -> Period
• Search -> Search options
• When -> Period

Also, I changed the "Select Dates" dropdown menu label, since it doesn't offer exact dates, but periods of time.

Before	After

I split these changes out of !1077 (closed) because this MR will likely need a design review.

@aday @snwh @bertob