Simplified runtime node type definition.

Merge branch 'master' into temp-runtime-node-def

Nodes: Improvements to edge panning in the node editor.

- New operator property to toggle edge panning in the keymap:
This is disabled by default to avoid edge-panning in cases where it
gets distracting, such as adding a new node. Only the explicit
translate operator(s) (GKEY or drag) have this enabled now.

- Restore the initial view rect on edge pan cancel:
The initial view rect is now stored in the edge pan operator data.
When an operator with edge panning is cancelled it can now call the
`UI_view2d_edge_pan_cancel` function to restore the original View2D
rect.

- Less delay in node editor scrolling:
Delay is useful when scrolling through long lists, such as in the
outliner, but makes node scrolling feel sluggish and unresponsive.
The lower scroll speed here makes a faster response the better option.

- Zoom influence feature:
Somewhat slower scrolling in UI-space when zoomed out. With the 0.5
zoom influence factor nodes behave as if zoom factor is halved,
otherwise it gets too fast when zoomed out. Previously scrolling would
always be constant-speed in UI space, now it's half-way between UI
space and node (view) space.

Bump scroll delay in node editor up to 0.5 seconds.

Merge branch 'master' into node-scrolling-improvements

Revert cleanup changes to node DNA.

This was used a some point to get a clear type for the node flags enum,
but isn't a necessary change right now. Node DNA cleanup should be done
in a separate patch.

Utility classes for compact node definitions in C++.

Node definitions in C++ are currently spread out over a large number of
files all over the code base (nodes, DNA, RNA, UI). By contrast, python
nodes are very compact and can be added much more easily. To make node
definitions in C++ more convenient, this patch adds some utility
functions that allow defining type properties and callbacks of a node
in a single file.

The NodeDefinition template takes a struct (which should usually also be
a child class of the template) and finds static fields and functions of
the template argument to use for the node type. The "Mesh Primitive
Ellipse" node has been added as an example case, it may not end up being
used. Other existing node types are unaffected, this patch only provides
alternative ways to define a node.

Known limitations:
- Only ID properties can be added in the node source files without
additional DNA structs. This should be sufficient for the vast
majority of nodes, but can be augmented with conventional DNA structs
in node->storage if necessary.
- Runtime node definitions are identified only by their idname, they do
not have a fixed integer type. This has to be taken into account for
versioning.
- Sockets are currently added in the init function. The "template"
system is not supported and a better alternative should be added
eventually.

Print function for new depsgraph nodes for debug.

Add a read-only flag to indicate depsgraph update.

The NODE_DEPSGRAPH_UPDATED flag indicates that a node has triggered a
depsgraph update. This can be accessed in a depsgraph "pre" handler
to find nodes which have changed through user actions.
The depsgraph clears this flag for the next update.

Removing links when disabling socket is not necessary.