Slideshow images loaded when needed (#241)

[JezerM]

Closes #241

Description

• Before these changes, slideshow images were loaded at startup, providing a static method to load them when required. Now, slideshow images are loaded when needed.
• At first, images path are stored statically. After slideshow-interval, i3lock loads the next image; if all images were read, then reloads the images path, allowing to add/remove/change these images.
• Also, only files that match an image type are stored in memory.

Release notes

Notes: Slideshow images are loaded when needed

[Raymo111]

LGTM