Add ability to create slideshows

[ermiry]

• Add the ability to select images to be displayed in a slideshow
• Ability to select the time of display for each individual image, as well as for the whole batch
• Press the space-bar to pause / resume the slideshow
• Press esc to exit the slideshow
• Ability to save slideshows to be displayed later in the same order
• Add side by side slideshows