Fix issues in create unit dialog

[lL1l1]

Issue

1: The create unit dialog does not use the standard way of loading icons, so all modded units do not have a proper icon when hovered over.

2: There are errors when mousing over templates with unloaded modded units.

warning: Error running HandleEvent script in CScriptObject at 279a5800: ...\lua\ui\dialogs\createunit.lua(1425): attempt to perform arithmetic on local `XSkirtO' (a nil value)
         stack traceback:
         	...\lua\ui\dialogs\createunit.lua(1425): in function <...\lua\ui\dialogs\createunit.lua:1420>
         	...\lua\ui\dialogs\createunit.lua(1446): in function `?'
         	...\lua\ui\dialogs\createunit.lua(1577): in function <...\lua\ui\dialogs\createunit.lua:1574>

3. There are errors when mousing over templates.

warning: Error running HandleEvent script in CScriptObject at 2c0a6100: ...\lua\maui\layouthelpers.lua(79): attempt to call method `SetValue' (a nil value)
         stack traceback:
         	...\lua\maui\layouthelpers.lua(79): in function `SetWidth'
         	...\lua\maui\layouthelpers.lua(104): in function `SetDimensions'
         	...\lua\ui\dialogs\createunit.lua(1479): in function `?'
         	...\lua\ui\dialogs\createunit.lua(1578): in function <...\lua\ui\dialogs\createunit.lua:1575>

Description of the proposed changes

Get the icon the standard way through UI Utils which is what is used for construction.lua.
Fix the errors.

Testing done on the proposed changes

Mousing over a modded unit in the create unit dialog gets the correct icon instead of falling back to the unit's albedo texture.

This green background is what fixing the mouseover creates by the way:

Checklist

- [ ] Changes are annotated, including comments where useful

• Changes are documented in the changelog for the next game version