Merge branch 'sev/luma_itemstime' into 'master'

Small icon updates for itemstime panel

I made a few revisions after the merge of the itemstime panel:

- Because HUD icons are inherited from the default theme, it is unnecessary to add identical icons to the luminos theme.
So I removed them to save some space.

- There already are icons for the strength and shield powerups, it is redundant to add second versions.
However, the quality of the new icons is a lot better than the old ones, so I moved
`default/item_shield` and `default/item_strength` to `default/shield` and `default/strength`.

- To match these new powerup icons, I updated the buff and superweapons icons.

- I added new icons to the Luma theme.

See merge request !169