[BUG] "show all" inconsistencies in the depot window

[Mariculous]

When your Depot window is in sell mode and the "show all" checkbox is checked, no vehicles will be shown.

How to reproduce?
1. Open the depot
2. Make sure there are some disassambled convoy in the depot. If not, buy some.
3. Select the "sell" mode
4. check the "show all" checkbox

Expected behavior:
Most consequently, show all vehicles, including those you don't own at the moment.
Alternatively, ignore the "show all" checkbox in sell mode.

[Ranran(retired)]

I think I have fixed this. Check the pull request # 296.