Ring-field focusing feature.

Summary:
Adds inner and outer limits to filter stars when running a full-frame focus procedure.
When option "Full Field" is enabled in the Focus tab, the procedure will first detect
stars that are visible on the frame, using either the SEP method (if selected) or the
Centroid method, and remove stars not in the ring surface bounded by the inner and
outer radius values before calculating the mean HFR.

Test Plan:
Basic items:

• Default options are "Sub Frame", and inner/outer radius boxes disabled.
• Selecting "Full Frame" enables the radius boxes, default values are 0% and 100%, respectively.
• Tooltips are coherent, and match their gadgets.
• Enabling "Sub Frame" disables "Full Frame", and enabling "Full Frame" disables "Sub Frame".
• Procedure values and selection are remembered from one execution of KStars to the other.

Procedure:

• Filter use is enabled with option "Full Frame".
• Filter is inoperant if inner/outer radius value are 0% and 100%, respectively.
• If one of the radius values is not the default, the filter is used.
• When the filter is used, two blue dashed circles are displayed on the focus frame (values 0% and 100% are not visible).
• Stars inside the inner circle are disregarded for the focus procedure, and are not marked by the star overlay.
• Stars outside the outer circle are disregarded for the focus procedure, and are not marked by the star overlay.

Reviewers: mutlaqja, wreissenberger

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D18510