Button to select between to split view or overlay
Create snapshots table and add a snapshot number column to images table
Several snapshots history and name saving

 + Write snapshot's history to xmp (TO BE FIXED)
 + Write snapshot's history to db
 + Write snapshot's name to db
TODO: we also need to read snapshots info from xmp file
Small modification in colums name in snapshots table
Make sure the functions are defined before using them
Create snapshots table and update history one in existing db
Read all the snapshots information from the db and store it in the sidecar
Fix several compile errors
Read snapshots' history from XMP sidecar files
Write the snapshot history to the db when it is taken
Init the GUI with the snapshots stored in the db
Show in the UI the snapshots taken.