Re-design the manifest editor

The re-design should take these points into account:

• The manifest editor has 3 icon placeholders for low, medium, and high DPI, but without hovering on them the user won't easily tell what those placeholders serve.
• There should be an option to clear icon selection 
• Would be good to mention/clarify permissions meant in "Include default permissions for Qt modules"  and "Include default features for Qt modules" 
• Allow search for permissions and better auto-completion. Currently, there's a simple auto-completion but it's not very efficient.
• Allow adding services from Manifest editor 
• Allow adding splash screen from Manifest editor
• Allow modifying android style value from Manifest editor