feat: add way to show app-icon if image data/path isn't providen

[JarKz]

Motivation

Need to follow D-Bus specification.

Changes

Added a way to show app-icon using freedesktop_icons crate with configurable icon theme and size. The app-icon will be shown if there is no image_data or image_path.

Syntax of icons configuration

[display]
icons = { theme = "Papirus", size = [ 64, 48, 32, 24] }

Note

The size presents possible values to pick and iterates from first to last until the suitable doesn't picked. Under 'suitable' I mean the first one that can be found in system resources.