unload slides and reload them when needed. this save A LOT of memory
an unused function crept in ca87d6e65bcf9fb3de7c433d3036aac2cea6f5e4
load slides only when required (a bit sooner) in the full-screen viewer too
load slides around the start index instead of the first slide (full-screen viewer)
prevent unloading a slide more than once
add selection actions to the context menu
Merge branch 'contextmenu'