add setting for default session handling

[linnea]

Define a start session behaviour: <blank|last|preselected> A command line parameter --session overrides this behaviour when used. If an application is started with a session file (e.g. post processing session file) as start up project, the different file ending induces a default project file, switches over to post processing part, and loads all properties from given session file.

[linnea]

start session behavior is implemented

tests with placeholders

todo: default placeholders
command line parameter
and the other part?

[ghorwin]

Milestone deleted

[ghorwin]

needs discussion