As part of this right-click select post, this adds some new theme colors related to grid rendering:
- Add grid theme color for node editor
- Call UI_view2d_multi_grid_draw with TH_GRID instead of a shaded TH_BACK, and color-blend TH_NODE_GROUP
- Movie Clip Editor's clip preview grid respects grid theme color
- Make ED_region_grid_draw use color-blended TH_GRID
- Add versioning code to allow fixing existing themes (the resulting themes should visually look the same as before)
These changes did cause some inconsistencies, even after adjusting the themes accordingly:
- In the movie clip editor, the alpha slider of the grid color affects the *background* and not the grid lines itself.
- In the movie clip editor, the grids used by graph and dopesheet mode could already be themed in the past. Now that the clip preview's grid can also be themed, two completely different modes share the same theme color.
TODO after this diff: Apply D8874 as well, so the add-on themes don't look out-of-place