As part of [this right-click select post](https://blender.community/c/rightclickselect/5ldbbc/), this adds some new theme colors related to grid rendering:
- Add grid theme colors for image/UV editor and movie clip editor
- Make `ED_region_grid_draw` use color-blended `TH_GRID`
- Add grid theme color for node editor
- Passes `TH_GRID` when rendering its grid via `UI_view2d_multi_grid_draw`
- Add versioning code to allow fixing existing themes (the resulting themes should visually look the same as before)
These changes did cause some differences in the UI, even after adjusting the themes accordingly:
- In the image/UV 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 (with alpha-channel!), two completely different modes share the same theme color. {F8808962}
TODO after this diff: Update themes in `addons/presets/interface_theme`