Allow display of sub-projects in project view

[nkoester]

I often use sub-projects, for example I got tasks in the projects something, something.sub1, and something.sub2.

Unfortunately, the project view (i.e. "Display mode: Project") of the project something does not allow me to also show the tasks of something.sub1 and something.sbu2. This would greatly allow to get an overview of the state of a full project and its sub projects.

I would propose to have a setting/checkbox to specifically show sub projects or something similar.

[DCsunset]

That sounds reasonable. It seems necessary to preprocess the project field in order to get the top-level name though.

I'm happy to accept any PR if anyone is keen on this.