Summary:
Currently, we use icons to display possible maximize button actions. The
problem with those icons is that they are confusing even for me, a person
who knows kwin in and out. The size_all icon implies that if you click the
maximize button, you'll be able to move the window around. Same goes with
the size_ver and size_hor icons, they indicate that user will start
resizing the window in the vertical and horizontal direction, respectively.

Test Plan:
Before

{F8312394}

After

{F8312395}

Reviewers: #kwin, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: ngraham, ndavis, broulik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29671