cc-background-panel.c: Added combobox for the background

picture alignment

Noticed that to change the background picture alignment, which is a very basic thing that you can easily change in any OS, was lacking in the control center, you need to download gnome tweaks. I'm new to contributing to the gnome project.

Added 2 comboboxes below the background and lock screen image buttons with the various options in the background.ui. Some of the code in cc-background-panel.c I based on cc-power-panel.c, and I'm not sure if anything should be done to avoid repeated code in these 2 files.