Feature: Implement Automatic Wallpaper Changer (Legacy)

[Aumaca]

Automatic Wallpaper Changer

I wanted to make it easier for users to set wallpapers that change automatically, so I made this contribution.

Features:

• Add "Next Wallpaper" to Panel Menu, that change to the next wallpaper (accordingly with the file in directory set by the user in filenames descending order);
• Add row "Change Wallpaper Automatically" to start changing wallpapers;
• Add row "Change Wallpaper Directory Path" to get wallpapers from a directory;
• Add row "Change Wallpaper Mode" to set the order to change wallpapers based in the filenames in directory (Sequential, Inverse Sequential and Random);
• Add row "Change Wallpaper Interval (minutes)" to change wallpaper based on time set by the user;
• Update .pot file and all .po files (pt_BR 100%);

[jeffshee]

@Aumaca LGTM, thanks for also implementing the feature for the legacy branch!