[effects/snaphelper] Do massive overhaul

Summary:
This patch fixes some of issues that the Snap Helper effect currently
has:

* If a window is being moved, there are visual artifacts (cause: missing
  addRepaint's);
* It uses addRepaintFull;
* For some reason, if a window goes deleted, it will be kept around as
  long as the Snap Helper effect needs it (visually, it doesn't look
  good).

Among other changes:
* Use variables to store color and width of grid lines;
* Use new connect syntax;
* Port to TimeLine;
* Fix coding style in some places.

Test Plan:
Tried the effect with different rendering backends, no longer see visual
artifacts when moving or resizing windows.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15695