PR#555 - A small step towards refurbishing the depot dialog

[Ranran(retired)]

I am currently working on updating the depot dialog for ver 15.
This is a necessary change due to the many parameters that will be added in ver 15 and involves a very significant change.

Pull request #555 is just a small preparatory patch, but it also serves as a small bug fix.

Currently, some themes have incorrect symbol designations. (This may have prevented the automatic generation of each theme pak.) This was a mixin of the planned symbols, even if they weren't implemented yet. Implementing this solves that problem.
The icon is a symbol that indicates comfort. It's only in the goods list at the time of this patch, but I plan to put it in many dialogs in the future. This is intended to save UI space.
There may be changes in many dialogs for ver15. This icon will be placed with it.

Some code cleanup in depot frame code.

Also, a chart showing the relationship between comfort and "max. Comfortable journey time" will be added to the goods list.
This is an attempt to reduce the importance of this notation in the depot dialog. At the same time, as reported in another thread, I have decided to submit a patch in advance of the depot dialog overhaul due to a possible bug in this display.

[jamespetts]

Excellent, thank you: now incorporated.