[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