Various snap fixes

[kenvandine]

• Use appstream meta data
• Specify prefix
• Added necessary plugs
• Define dbus slot
• Dropped unused stage-packages
• Set HOME to SNAP_USER_COMMON to avoid versioned runtime data

[hellsworth]

I built this locally and installed it. It ran fine and I imported a folder with 400 pictures and rotated a few.. seems to run smoothly!