Add duration to window activities

[SRichner]

Currently, we only store the timestamp of the switch to a new window/application. With every window activity, we want to check the latest entry and calculate the duration of the previous activity and store it in a separate database column as well.

TBD: this issue needs to be specified in more detail

Some notes:

• this will require updates to the UserInputTracker, see: https://github.com/HASEL-UZH/PA.WindowsActivityTracker/issues/1
• @SRichner and @royru created respective workarounds for AIRbar that we could consider