Restructure data import

[jscheidtmann]

Currently 3 imports need to be done in the right order:

1. Load fits data from a directory
2. Load ImageMetaData from the session Plugin
3. Load PHD guidelog

Instead:

1. Provide a starting directory by user
2. scan for fits, session meta data, PI SubframeSelector data, Siril equivalent, ...
3. scan a configured PHD directory for matching guidelogs