As part of [this rightclickselect 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 alpha-blended `TH_GRID`
- TODO: should this use color-blending with the window background color instead?
- Add grid theme color for node editor
- Passes `TH_GRID` when rendering its grid via `UI_view2d_multi_grid_draw`
- Add alternating row theme color for file browser
- Make `draw_background` use color-blended `TH_ROW_ALTERNATE`, similar to outliner
- Bump subversion 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. Should the grid background be a separate theme color instead, and remove the alpha channel from the grid theme color?
- 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. Should they maybe be split into two separate theme colors (e.g. "Timeline Grid" and "Clip Grid")? {F8808962}
- When inspecting nested groups in node editor, the grid keeps its default color instead of using the "node group" color (which was the old behavior). This makes the grid higher contrast even with the default theme. It should maybe be blended with the node group color as well? {F8808967}
TODO after this diff: Update themes in `addons/presets/interface_theme`