Summary:
This patch ports the Effects KCM to QQC2, which yields the following benefits:
1. General performance enhancements of only using QQC2 rather than a mix of 1 and 2
2. Some code simplification is possible
3. Improves the appearance of the checkable menu items in the dropdown menu, fixing https://bugs.kde.org/show_bug.cgi?id=402701
4. Improves the appearance when using a fractional scale factor, fixing https://bugs.kde.org/show_bug.cgi?id=396725
5. Fixes the incorrect size when opened from `kcmshell5`

Along the way, two visual changes are introduced as a by-product of porting:
1. The scrollbar is inline, so it overlaps some of the buttons in the content. This is somewhat undesirable, but adopting a Kirigami scrollview would fix this.
2. The button that displays a dropdown menu no longer has a downward-pointing arrow to indicate as such. This is not my preference, but there's a benefit to being consistent, and eventually we could can change this in one place to impeove the appearance of buttons that display dropdown menus everywhere in one fell swoop.

BUG: 396725
BUG: 402701
BUG: 396076
FIXED-IN: 5.15.0

Test Plan:
All functionality still works

In System Settings, showing improved checkable menu item appearance:
{F6523385}

In `kcmshell` at 1x scale:
{F6523386}

In `kcmshell` at 1.5x scale:
{F6523387}

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17928