[OTHER] changed all files permissions
[CORE][DB][UI] album files loaded when clicking on previews
[CORE] implementing concurrent thumbnail building
[CORE] almost complete preview adding
[CORE][UI] thumbnails correctly added to ImageViewer
[CORE][UI] thumbnails correctly removed when closing ImageViewer
[CORE][UI] created custom QLabel PreviewItem
[CORE][UI] added file name to PreviewItem... going to create custom QImage
[CORE][UI] no need to create custom QImage, the file path is stored in QImage.text() and in PreviewItem
[CORE][UI] implemented selection/unselection mechanism
[CORE][UI] first preview created is shown
[CORE][UI] fixed a few bugs when selecting images
[UI] fixed ImageViewer behavior when resizing window
[CORE][UI] ImageViewer controllable by keyboard
[CORE][UI] first preview marked as selected only when QFutureWatcher::finished()
[CORE][UI] preview scroll area moves to selected item if not visible
[CORE][UI] fixed crash due to PreviewItem::setSelected() when closing viewer while loading thumbs