Improve comprehensibility and consistency of window placement mode names

Summary:
Right now a few of the window placement modes suffer one or more of the following problems:
- Inconsistency between the text shown in the Window Behavior and Window Rules KCMs
- Title does not indicate what it does
- Awkward wording
- Lack of unity in grammatical mood between the different modes

This patch fixes these issues by improving the strings and standardizing on the descriptive mood:
- Smart -> Minimal Overlapping
- Maximizing -> Maximized
- Cascade -> Cascaded
- Zero-Cornered -> In Top-Left Corner

Strings are also unified between the window behavior and window rules KCMs, and docbooks are adjusted accordingly.

Test Plan:
{F7279685, size=full}
{F7279694, size=full}

Reviewers: #kwin, #vdg, ndavis, filipf, zzag, romangg

Reviewed By: #kwin, #vdg, ndavis, filipf, romangg

Subscribers: gladhorn, alexeymin, IlyaBizyaev, broulik, hein, hpereiradacosta, GB_2, alexde, davidedmundson, romangg, cfeck, filipf, zzag, ndavis, kwin, kde-doc-english

Tags: #kwin, #documentation

Differential Revision: https://phabricator.kde.org/D23415