Make all dragging actions abortable with right click

[Keavon]

This is an ongoing list of all actions we find which need to be made abortable by right-clicking. Right clicking should be possible anywhere in or outside the app window, not just over the widget that's being dragged.

Header bar and widgets

• Menu bar entries should—first of all, open on mousedown and let you drag down and release on an entry—then right click in the menu to abort and close it
• Same as above for dropdown menus

Layers panel

• Dragging to rearrange layers (in Firefox, this actually confirms the action instead of aborting it, in Chrome it just does nothing)

Color picker

• Saturation/Value square
• Hue track
• Alpha track
• Gradient spectrum input knobs

Viewport

• Scrollbars

Tools

• Select & Path tools: drag-selecting, upon right clicking, should restore the selection before drag-selecting

Node graph

• Box selection should, upon right clicking, restore the selection before drag-selecting
• Dragging nodes should restore their initial positions
• Dragging a wire should restore its initial connection