Provide optional process patterns for processes denoting inactivity

[marcus67]

Current behavior: There is only one process pattern to recognize activity of a user. If at least such a process is found the user is active, other inactive. Active processes are terminated according to the configured rules. In order to comply with the rules the user usually has to logout completely resulting termination of all processes. This is disadvantages is an activity cannot be interrupted and thus cannot be resumed in the next session.

Desired behavior: There are two more process patterns:

• prohibited processes: if a process matching this pattern is found the process is terminates immediately. This allows the creation of user accounts that have a flexible time schedule (e.g. for schooling) but should not have access to certain process groups (e.g. games). This part is covered in https://github.com/marcus67/little_brother/issues/129
• processes denoting inactivity: if a process matching this pattern is found the user is regarded as inactive although there may be a large number of other processes active. The pattern can be configured to match lock screen processes (by full binary path name). This way the user can log out without losing its activity context.

[marcus67]

Using a process-pattern for inactivity will probably not work since the process doing the actual locking is started at the start of the session but remains inactive until the screen needs to be locked. In case of mate the binary is called mate-screensaver. It comes with a control tool called mate-screensaver-command which allows the state of the screensaver to be queried. So, instead of searching for processes of a given pattern it makes more sense to call the control-binary regularly and check the state. However, this needs to be done on the client, hence we also need to change LittleBrotherTaskBar.

[marcus67]

See https://github.com/marcus67/little_brother_taskbar/issues/34