Moving anchors together with the object

[QbieShay]

Godot version:
11d7738

Issue description:
The only way to have an object resize proportionally with the screen (same behaviour as % in CSS) is to move the anchors around and have always margin 0.
The current workflow for this kind of task is clunky as there is no way to move the anchors together with the object in the 2D view.
Related to #1276

Proposal
I don't have a clear idea yet on how this feature should look like, as i foresee many problems for when people just want to move only the margins.
I'd like to discuss here and see if there is a nice solution to this problem
@groud @JFonS I'd like your opinion on this :)

[groud]

Something like that probably makes sense, but like you, I am not sure how this feature would look like.
Maybe something like a "lock margins" mode that would move the anchors instead of the node.

[JFonS]

I'm not too familiar with the anchors/margins model. But an option could be to add toggle in the "Layout" menu (in the viewport toolbar). When it's enabled, dragging the 8 points edits anchors instead of margins and size.

[groud]

@JFonS note that one objective was to move the Layout menu to the inspector at some point. So that might not be the best solution to move it there. But indeed the feature would be the same. :)