UI: New button/widget type for Asset Browser like preview tiles

This button type shows a preview image above centered text, similar to
the File Browser files in Thumbnail Display Mode or the default Asset
Browser display. In fact we may want to port these over to use the new
button type at some point.

Will be used by the asset view UI template that will be added in a
following commit. That is basically a mini version of the Asset Browser
that can be displayed elsewhere in the UI.

source/blender/editors/include/UI_interface.h

@@ -367,6 +367,9 @@ typedef enum {
   /** Buttons with value >= #UI_BTYPE_SEARCH_MENU don't get undo pushes. */
   UI_BTYPE_SEARCH_MENU = 41 << 9,
   UI_BTYPE_EXTRA = 42 << 9,
+  /** A preview image (#PreviewImage), with text under it. Typically bigger than normal buttons and
+   * laid out in a grid, e.g. like the File Browser in thumbnail display mode. */
+  UI_BTYPE_PREVIEW_TILE = 43 << 9,
   UI_BTYPE_HOTKEY_EVENT = 46 << 9,
   /** Non-interactive image, used for splash screen */
   UI_BTYPE_IMAGE = 47 << 9,