ui: add 'winblend' to support blending of floating windows

Also add `hi blend=` attribute to override transparency of indiviual attributes.

src/nvim/highlight_defs.h

@@ -25,6 +25,7 @@ typedef struct attr_entry {
   int16_t rgb_ae_attr, cterm_ae_attr;  ///< HlAttrFlags
   RgbValue rgb_fg_color, rgb_bg_color, rgb_sp_color;
   int cterm_fg_color, cterm_bg_color;
+  int hl_blend;
 } HlAttrs;
 
 #define HLATTRS_INIT (HlAttrs) { \