Merge mode should be clear to the user

[ceesem]

Unlike the clear user actions for split mode (at least for multicut), merge mode is barely visible to the user, only causing a small low-contrast change in the notification bar at the bottom of the screen when it is entered and when the first object is selected. Indeed, if not noticed immediately these notifications go away and there is no information available to the user that the next click will cause a lasting edit to the segmentation. We have already had one event where a modestly experienced proofreader never realized they made a merge operation and were confused by the change in the data that occurred. This is particularly easy to do, since m is next to the keys for moving through sections. Making this interaction much more visible to the user would help with this.

1. Entering merge mode to begin with should have some unmissable visibility, like changing the cursor or something similar.
2. The points that are to be merged should be clear to the user. For example, initiating a line annotation (like point annotations are used for splits) would be a nice visual cue. Similarly, maybe control-clicking when already in merge mode would be an easy way to ensure that the user is acting with intention while still fitting cleanly within the usual grammar of the neuroglancer.
3. A notification display that indicates how to cancel merge mode without doing a merge would also be useful.