Floating toolboxes and Designer top menu

[tznind]

First raised by @TonyHenrique in #63 (comment)

The idea is to add floating toolboxes and a menu at the top of the screen for accessing Designer commands. Something similar to this:

Currently commands are all accessed via shortcuts or context menu (right click) and the users view (being edited) has full screen realestate. This is particularly useful for low resolution devices (see #39 )

We need to develop this carefully if it is accepted as the way to go and not merged until it is really good. Perhaps with a feature toggle to switch between modes.