Bugfixes, folder dialog improvements & ARM support on Linux

Improvements

• Hide hidden folders when showing subfolder in folder import
• Add progress bar when scanning large folder for files to import

Fixes

• Fix crash when trying to import a root directory
• Fix crash when a dropped item has no file on disk (an entry dragged out of a ZIP folder, a mail attachment), and import its contents instead
• Limit modal window size so it cannot be bigger than the screen size