Fix allows slots with default values to be set to None

[xotmatrix]

This patch addresses a problem with setting certain slots to be empty.

Background:
Attempting to select "—None—" for a slot with a default device will immediately select the default device. I do not have a Mac to compare to, but according to my discussion with Kelvin Sherlock, the correct behavior is to explicitly set the slot to empty in the command line. For example, slot 6 defaults to diskiing when no command line argument is given. To disable it, the empty string -sl6 '' is required. Ample does this, but AmpleWin does not.

Cause:
This problem is caused by initialize_default_slots() being called each time the UI selections change. That function will automatically set a slot to its default value because an empty slot is indistinguishable from an unset slot.

Fix:
According to Kelvin, Ample does not initialize the system when the UI is changed. It simply updates the command line. The first part of this patch removes the reinitialization when the UI is refreshed. The second part sets the slot to a quoted, empty string whenever it has a default=False value in its XML, which I believe means the slot must be explicitly disabled in the command line.

Linux:
It looks like the same fix could be applied to the Linux port. I cannot build for Linux, so I left this alone. I think these are the equivalent lines.

ample/AmpleLinux/main.py

Line 1290 in d6ef2a9

# 0. Re-initialize defaults for any newly appeared slots/devices

ample/AmpleLinux/main.py

Line 1414 in d6ef2a9

item.setData(opt['value'], Qt.UserRole)