Capitalize background layer name in remaining cases

A while ago there was a commit (can't seem to find it) that capitalized the background layer name. Although this works when creating a new document, it doesn't seem that this fix was applied to the following cases: drag-and-dropping an image into Krita, pasting the clipboard into a new image, and "Open With"ing an image with Krita. This commit capitalizes the remaining cases. It also adds a missing space in the layer name when pasting into a new layer.

Test Plan

Observe the background layer name when drag-and-dropping an image into Krita, when pasting the clipboard into a new image, and when "Open With"ing an image with Krita. Observe layer name when pasting into a new layer.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.