regression: custom shortcuts are reverted to defaults upon restart

[Cobra]

It's not possible any more to set custom shortcuts.

The shortcut can be changed in the settings dialog and is still there when reopening it. But after restarting josm to use it, the custom shortcut is replaced by the default one.

This will definitely break the workflow for many users having set up their own shortcuts (which mostly the old ones you remembered, like alt-b for adding a tag), so I set this to critical. Sometimes this will have unforseen and unwanted consequences when you fire some other action instead of the intended one.

This will happen even with a blank preferences.xml.

[stoecker]

In [4900/josm]:

fix #7358 - custom shortcuts broken