Unify sidebar and main view type

[ysmilda]

From discussion #119 it became clear there is a wish to be able to use the different object types in any position.

The proposal is to

• Combine the sidebaritem and mainitem into one
• Combine the modals for both
• Allow to drag and drop items from sidebar to main and vice versa

This issue is made to work out a plan of attack as well as track the design decisions along the way.