Marker Creation - do not provide a default text to label

[mcwhitaker]

When creating a new marker, a label text should be initially blank until the user provides a text. Currently the label text starts as Untitled Label.

Also, when we click on the marker annotation to edit it, the cursor should be placed in the label field, not the description field.

Possibly related to #282

[cjcolvar]

@mcwhitaker Avalon team can take care of this one.