#70 Expose Merge function in GUI

[k_uber]

Although Twinkle supports database merging, this feature is hidden.

To merge now:
Using Details->Rename, rename the source database as the name of the target database. A dialog will show asking for confirmation of merge. Columns should be consistently named, or new columns are added.

Desired bare minimum:
add text to rename dialog along the lines of "Use existing database name to merge this database into named database"
Nicer:
Merge menu/tool item with database pick list.