Use platform native file browser instead of tkinter.filedialog

Currently file and directory selection is handled via the `filedialog` class, which is serviceable and portable but offers a degraded experience (IMO) from the OS native file browser. As an example, on Windows I keep my sketches at a location similar to: `E:\shared_home\Documents\Git\\\`, but also have the examples and other code within some default and/or sensible locations on `C:\`. Switching back and forth between the two locations in the current dialog requires clicking **This PC** and navigating down from the drive root. Invoking explorer.exe (or xdg-open in Linux, or the OSX equivalent, etc) would allow me to browse much faster via the side panel's Quick Access links or tree view, or whatever analogous customizations users have set up on other operating systems.

Granted this increases fragility when supporting as many platforms as possible, but it's a solvable problem.

Start by locating the filedialog class and the code paths responsible for file and directory selection. Compare the platform-specific options mentioned in the issue—explorer.exe on Windows, xdg-open on Linux, and the OSX equivalent—and define done as native browsing working across the supported platforms without breaking selection behavior.

Written by the indexing model from the issue text.