keyboard: New and updated symbolic assets

• renamed all assets to start with osk- disambiguate them from other keyboard status icons
• updated the design of hide, enter and keyboard layout icons to be more rounded
• new symbolic assets for emoji and delete actions to move away from using 16x16
• updated keyboard scss to use the scalable icon definition
• updated the zwnj asset to align with GNOME icon style
• changed all references to icons in layouts