TextGridNavigator & TextGrid: Add search tier...

A command to extend the search of the selected TextGridNavigator with another tier of the selected TextGrid.

## Settings

Tier number
  defines the tier of the TextGrid that will also be used to limit make the searches more specific. This tier has to be different from the tiers that are already in use.
The navigation context for this tier has to be defined.
Match domain
  defines the start and end times of the match domain on this tier. Various options exist, the default, Topic start to Topic end takes the start time and end time of the interval that matches the topic on this tier.
Match domain alignment
  defines the alignment between the match domain in this tier to the match domain in the topic tier. For example, if the option overlaps before and after were chosen and the match domain in the selected tier is [tmin2, tmax2] while on the topic tier it is [tmin1, tmax1] then a match would only succeed if tmin2 < tmin1 and tmax2 > tmax1.