UI: Add initial "grid view"

Part of T98560.
See https://wiki.blender.org/wiki/Source/Interface/Views

Adds all the basic functionality needed for grid views. They display
items in a grid of rows and columns, typically with a preview image and
a label underneath. Think of the main region in the Asset Browser.

Current features:
- Active item
- Notifier listening (also added this to the tree view)
- Performance: Skip adding buttons that are not scrolled into view
  (solves performance problems for big asset libraries, for example).
- Custom item size
- Preview items (items that draw a preview with a label underneath)
- Margins between items scale so the entire region width is filled with
  column, rather than leaving a big empty block at the right if there's
  not enough space for another column (like the File and current Asset
Browser does it).
- "Data-View Item" theme colors. Not shown in the UI yet.

No user visible changes expected since the grid views aren't used for
anything yet.

This was developed as part of a rewrite of the Asset Browser UI
(`asset-browser-grid-view` branch), see T95653. There's no reason to
keep this part in a branch, continuing development in master makes
things easier.

Grid and tree views have a lot of very similar code, so I'm planning to
unify them to a degree. I kept things separate for the start to first
find out how much and what exactly makes sense to override.

1 files changed, 9 insertions, 0 deletions

diff --git a/release/datafiles/userdef/userdef_default_theme.c b/release/datafiles/userdef/userdef_default_theme.c
index 19798e3f165..04ad04d4812 100644
--- a/release/datafiles/userdef/userdef_default_theme.c
+++ b/release/datafiles/userdef/userdef_default_theme.c
@@ -194,6 +194,15 @@ const bTheme U_theme_default = {
       .text_sel = RGBA(0xffffffff),
       .roundness = 0.2f,
     },
+    .wcol_view_item = {
+      .outline = RGBA(0x2d2d2dff),
+      .inner = RGBA(0x303030ff),
+      .inner_sel = RGBA(0x4772b3ff),
+      .item = RGBA(0x4772b3ff),
+      .text = RGBA(0xccccccff),
+      .text_sel = RGBA(0xffffffff),
+      .roundness = 0.2f,
+    },
     .wcol_pie_menu = {
       .outline = RGBA(0x242424ff),
       .inner = RGBA(0x181818ff),