Consolidate footers
Right now the page can have a footer with a combobox on it, and the dialog that contains it can have a footer with an Upload button on it. When both are shown at once--for example when getting new wallpapers--the dialog shows two stacked footers each with only one thing on it, which isn't ideal.
This comit fixes that by combining both footers into one in the page content; now the dialog has no footer of its own.
There are no visual changes anywhere except the wallpaper dialog.
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