Use private inheritance where applicable
Code reorganization: move applicable files to widgets or models folders
Changed return type of MainWindow::openFile from void to bool
Move FileSystemModel class code to separate file and create LocationWidget from code in MainWindow class
Don't use separate folders for widgets with few files
Move default zoom sizes from Helper to ZoomWidget
* Remove quit() and boss() signals from PictureItem and add mouseWheel, mousePress  and mouseDoubleClick signals. Move some event handling logic from PictureItem to MainWindow.  * Simplify some PictureItem functions
Remove following settings options: "After Scrolling to Bottom, Scroll by Width", "Change page after scrolling to end", "Jump to end of page after scrolling up", "Prevent changing pages for:". Some of them may be back in future versions.
Move mouse wheel handling from PictureItem to MainWindow. Remove following signals from PictureItem: pageNext, pagePrevious, setFullscreen, zoomIn, zoomOut
Remove FileInfo::rarImagePath , functions that used it already converted directory separators so they can use FileInfo::zipImagePath instead.
Rename FileInfo::zipImagePath to FileInfo::getArchiveImagePath and FileInfo::getZipPath to FileInfo::getArchiveContainerPath
Settings dialog: Show Hardware acceleration checkbox as disabled in Qt5 builds
Reorganize MainWindow menus. Add more shortcuts (+ for zoom in, - for zoom out, 0 for reset zoom).