Add multiselect to asset dock

[aidandavey]

Allows selection of multiple mesh assets in the dock for instancing or removal.

Always keeps at least one asset selected.

Click tile to select/deselect

[Xtarsia]

Theres a few issues testing this:

Having more than 1 mesh asset, and then trying to remove the last asset when not currently selected, forces it to be selected. Which then causes errors when painting assets.

Having CrystalC as asset ID 0, Rock A as ID 1, have both selected, then add a 3rd asset, Rock B, causes ID 0 to render as Rock B, along with the thumbnail to swap to Rock B, even though CrystalC is still the chosen Scene file in the inspector.
Saving and reloading didnt restore the rendered meshes to the correct state, even though the Scene file for ID0 was still showing as CrystalC.
Re-loading CrystalC for ID0 results in ID2 becoming CrystalC.
This issue seemed to only get worse as the number of meshes inceased.

When it did work, it was nice to use, I can see it being very useful!

[aidandavey]

Theres a few issues testing this:

Having more than 1 mesh asset, and then trying to remove the last asset when not currently selected, forces it to be selected. Which then causes errors when painting assets.

Having CrystalC as asset ID 0, Rock A as ID 1, have both selected, then add a 3rd asset, Rock B, causes ID 0 to render as Rock B, along with the thumbnail to swap to Rock B, even though CrystalC is still the chosen Scene file in the inspector. Saving and reloading didnt restore the rendered meshes to the correct state, even though the Scene file for ID0 was still showing as CrystalC. Re-loading CrystalC for ID0 results in ID2 becoming CrystalC. This issue seemed to only get worse as the number of meshes inceased.

When it did work, it was nice to use, I can see it being very useful!

Both of these issues have now been addressed, thanks!