ui/virtualconsole: fix sorting in the level tab of the slider dialog

[jpue]

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#1722 (or 4bbba7ba85716ce3076739db013a25633a9dcaac, respectively) introduced the possibility to sort the fixture-channel-list on the Level page of Virtual Console Slider properties dialog.

However, this was done by comparing the channel names as a string and thus introduced known "sorting anomalies": Channel 1 - Channel 10 - Channel 11 - Channel 2 - ... (tested with the Lixada Mini Gobo Moving Head fixture).

In my opinion, this is particularly unintuitive for non-programmer users of QLC+.

This PR attempts to fix this problem by basing the sort on the numeric channel id instead of the string name.

(I am not very familiar with Qt Framework and its internals, so if there is a better solution, please treat this PR as an issue report and ignore my code.)