Introduce the linear focuser

Summary:
Linear focuser is a simple autofocus algorithm that always scans in,
sampling at regularly spaced intervals. It may be slower than the
other algoriths, but the hope is that by minimizing direction changes
backlash will be minimized and the algorithm may provide more stable
autofocus results. This change also introduces an interface that hides
the implmentation of the autofocus algorithm, and future changes may
moved the other existing algorithms into this framework.

Linear focuser mainly works well when started near the ultimate focus position.
It depends on the step-size, max-travel, and tolderance parameters, which should be
chosen well. It may take more iterations to find focus than the other algorithms.

Test Plan:
Like other autofocus algorithms, use the simulator and set the position and step-size such that
the best focus is within 5 or so steps from the current position. It should then find focus.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: yurchor, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D26075