Unify some providers options

[Eloy García Almadén]

In special, options related to the resolution of the wallpapers downloaded. The GUI could have a common part within the providers tab to set:

• Download only wallpapers which fit the native screen resolution
• Download any wallpaper and keep the original resolution
• Download any wallpaper and resize it to the screen resolution

Maybe if the GUI has these three options, All resolution checkbox and custom resolution boxes don't have sense anymore

[Eloy García Almadén]

Done!