0.8.0

Main changes:

● Now it's possible to drag all selected items, including directories.
● Added an option for only expanding top level directories when loading archives.
● Whether the option "Fit first column into available width" is checked or not, the first header column is expanded to fill the available width.
● Added a workaround for Qt's problem with scrolling to deep non-expanded tree items.
● Don't show the header without content.
● Resize columns appropriately on expanding/collapsing a directory item.
● Added application/x-xz, application/x-bzip and application/zstd to the supported extractions by using 7z.
● Show the compressed size prefixed by ≥ if the file size is missing.
● Added cmake support (in addition to qmake).