FileDialog Improvements: Crash Fix, File Filtering, Permission Checking

[Bamboy]

Another PR to improve FileDialog:

• Fixes issues FileDialog -> AccessDenied Exception will crash #443 FileDialog / MyraPad: Crash on Linux (Access denied) #492
• Adds a file/folder filtering system based on FileAttributes enum.
• Files and Folders will now not display at all if the app (or class settings) does not have permissions to interact with them. Even if you DID (like by overriding) it will now show a popup with a basic message about the IO exception. (popup untested)
• Read-only files are filtered out in "SaveFile" mode.
• Non-directory "files" are filtered out in "ChooseFolder" mode. (technically, redundant with prior code)
• Added ability to show system-hidden files and folders via public variable (default off).
• Files and folders are now sorted alphabetically instead of randomly by the runtime.
• Linux: Added a missing label for the primary OS partition, and it now explicitly directs to / instead of /home, or another mounted directory.