Make the screensaver mode more user friendly

[nvllz]

Second attempt in making the screensaver mode much more user friendly:

• set the slide delay to every 30s
• reduced the range of movement including random offset
• made the animation smoother

The offset range is related to the screen width, ensuring that movement is slight and not distracting.

df670d53-c365-4d1e-b488-174675e6b86e.mp4

Related to #154, #318

[adrcotfas]

Thanks for your contribution! I will include this in the next release.

[adrcotfas]

This was included in the latest release.