Draft mode for node tree

Created by: portnov

Enabled/disabled by a toggle button near "Update tree".
Input nodes, such as "A Number", have (and store) separate value for Draft mode (which is usually smaller, but it depends). It is displayed and used instead of main value, when the tree is in Draft mode.
Some nodes have (and store) separate values of their settings for Draft mode.
All nodes have programmatic access to this mode (smth like if self.tree.draft:). In draft mode, complex nodes execute some simplified version of their algorithm.