kcm/nightlight: Move back to "display" category
Reason for the change
The theme category is for themes, and is the last place a user would look for when looking for night light settings. This is kind of backed up by confusion when I was setting up a new device a few days ago, but I believe this would hold up if you run a user test (ask a user, new or old, to configure the blue light filter from System Settings).
This KCM was moved from display to themes in 4b84bed6 implementing systemsettings#15 (closed), presumably because Night Light was called Night Color back then and thus deemed to be a "Color And Themes" member. I believe this to be a mistake: This display-tinting feature is not seen as color customization, as evidenced:
- by its own rename to Night Light,
- by the initial decision to place Night Color under "display",
- and by the fact that a blue light filter is under equivalents of the display category on literally every other platform
This mismatch can also be seen by the fact that every single page under themes involves selecting a theme of some sort, with Night Light being the only exception.
Test plan
Open System Settings and check to see Night Light under the "Display and Monitors" category.
To check this change has the desired effect: ask a user (non-Plasma user, existing Plasma user, Plasma dev) to configure the blue light filter from System Settings. Before the change, I expect lots of confusion. After the change, I expect less confusion.