Allow to select "No tasks app" to disable task sync even if a tasks app is installed

[rfc2822]

Scope

• I'm sure that this feature request belongs here and not into Discussions.

Description

See #102

We should probably add a "No tasks app" option in App settings / Tasks app.

This probably means that we shouldn't automatically select the tasks app when DAVx5 detects that

• no tasks app is currently selected, and
• a supported tasks app is installed.

Further info

Users may want to use their tasks app as a stand-alone app, without DAVx5 integration.

[sunkup]

This probably means that we shouldn't automatically select the tasks app when DAVx5 detects that

• no tasks app is currently selected, and

• a supported tasks app is installed.

We don't want to disable this completely though, right? Only after a user actively decides they don't want any tasks integration, the automatic task app selection should be disabled from then onwards.