Integrate exotic file extensions and uppercase equivalent of all possible files to select with dialogs. Code cleanup
Cleanup code to integrate multithreading process for dialogs into the DialogToolkit (avoid poluting UserInterfaceManager and improves reliability)
MultiFileSource plays a sequence of numbered images.
Discarding use of ZENITY under linux (previously used with the tinyfiledialog) because snapcraft makes it impossible to use :(. Reimplementation of GTK+ dialogs directly inside vimix code. Note: no changes for OSX. Complete cleanup of cmake file.
