Asset Gallery - improvements to make more useful

[Jaqster]

Should investigate usage of the Gallery tab in the Assets Editor.
Unclear what this is being used for, or if it's being used at all.

To make it more usable for adding images to a project (i.e. importing an asset pack), we could make a few improvements here:

• Add some sort of filtering at the top to make it easier to navigate (filter by asset type or source extension)
• Only have 1 button to "Import to Project"