screen: use dedicated message grid

add proper msg_set_pos event, delet win_scroll_over_*

make compositor click through unfocusable grids

add MsgArea attribute for the message/cmdline area, and add docs and tests

src/nvim/highlight_defs.h

@@ -93,6 +93,7 @@ typedef enum {
   , HLF_INACTIVE    // NormalNC: Normal text in non-current windows
   , HLF_MSGSEP      // message separator line
   , HLF_NFLOAT      // Floating window
+  , HLF_MSG         // Message area
   , HLF_COUNT       // MUST be the last one
 } hlf_T;
 
@@ -146,6 +147,7 @@ EXTERN const char *hlf_names[] INIT(= {
   [HLF_INACTIVE] = "NormalNC",
   [HLF_MSGSEP] = "MsgSeparator",
   [HLF_NFLOAT] = "NormalFloat",
+  [HLF_MSG] = "MsgArea",
 });