machine-properties: also set default on key_release

When using the keyboard to step modify values, we also want to update the
default after an edit.
machine-properties-dialog: replace 'selectable' hack for param label

We don't want to actually select the label. This was just a cheap way enable
events, so that the context menu works. Instead use an event_box. Also focus
the main widget when clicking the event-box.
machine-properties-dialog: replace 'selectable' hack for value label

Apply a simillar change like in the previous commit for the value_label, if any.
machine-properties-dialog: fix typo in quark var name