Change Extension Manager "Install from file" to use NSOpenPanel

[wjk]

While the GTK-based file browser dialog is still usable, it will cause problems when used on macOS Catalina. The Cocoa-based NSOpenPanel has special exemption from the privacy protections for the Desktop, Documents, and other folders. If the GTK-based file browser is still used, the user must permit VSMac to access the folders when prompted by macOS, or else spurious permission-denied errors will be returned from the filesystem. (If the user denies permission by mistake, they must go into Privacy Preferences to undo the change, a needlessly complex process.) Thanks!

VS bug #950510

[Therzok]

The panel is implemented in mono-addins. Let's track this here though.