power: Correctly lookup or insert new items into combobox

The code to lookup or insert items into the combobox had a few issues.
It would assume that the items are sorted, causing existing items to not
be found and be inserted instead. It also would simply forget to insert
an item if it was larger than all existing items.

This code is now changed to iterate over all items, finding the best
insertion point in the process (next item has a larger value, or the
values are not increasing anymore). The item will only be inserted if it
has not been found.

Fixes #261
11 jobs for benzea/power-combobox-value-setting in 1 minute and 43 seconds (queued for 3 seconds)
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passed #142661
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