Categorise arguments to ensure user-specified ones take precedence
over values set by the launcher.
When any positional arg is set, exclude all positional ones from being
set at all, to prevent strange interactions.
This simplifies the launcher's rendering since the widget-based
code is gone, various things get hooked up as a result. Styling
fixes everywhere as well.
Menubar on Linux/Windows needs to be re-added.
Updating the value takes immediately, as it did in 2017.1 and prior.
Additionally, passing —download-dir on the command-line is detected
and handled specially; the UI option is disabled, and no changes are
made to the value passed in. (The launcher does not override the value
with any value it has saved)
