TE: Static Meshes tab does not keep boxes checked after reloading lost wad

[Castigavi]

Summary: It seems that when a wad is lost and so builder needs to specify path to the wad again (by using Browse button), then in Static Meshes tab all slots have unticked boxes. This results in losing setup of some objects that builder wanted to merge with level geometry. There is no way to restore setup except for manually ticking the boxes again.

Since TE remembers placement on objects and just shows them as "missing", it'd make sense and feel consistent if Static Meshes tab also remembered object setup even when wads are lost.

Repro steps:

1. Create a project and load wad with several statics.
2. Go to Static Meshes tab and tick several checkboxes (eg. merge with geometry and intepret shades as effect).
3. Move wad somewhere else, so TE can't find it.
4. Enter Static Meshes tab again and notice how it is empty.
5. Use Browse button in Object Files tab to find the wad.
6. Return to Static Meshes tab and notice how all boxes are unchecked.

Proposed way of handling it: If it's possible I'd check if an object is placed on the map. If so, then its slot would stay in Static Meshes tab (with boxes checked) even when a wad is lost. If not and the slot is not present in any of the loaded wads, the slots would be removed.