Introduce nvim namespace: Move files.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

src/nvim/api/tabpage.c

@@ -0,0 +1,88 @@
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "api/tabpage.h"
+#include "api/vim.h"
+#include "api/defs.h"
+#include "api/helpers.h"
+
+int64_t tabpage_get_window_count(Tabpage tabpage, Error *err)
+{
+  uint64_t rv = 0;
+  tabpage_T *tab = find_tab(tabpage, err);
+
+  if (!tab) {
+    return rv;
+  }
+
+  tabpage_T *tp;
+  win_T *wp;
+
+  FOR_ALL_TAB_WINDOWS(tp, wp) {
+    if (tp != tab) {
+      break;
+    }
+    rv++;
+  }
+
+  return rv;
+}
+
+Object tabpage_get_var(Tabpage tabpage, String name, Error *err)
+{
+  Object rv;
+  tabpage_T *tab = find_tab(tabpage, err);
+
+  if (!tab) {
+    return rv;
+  }
+
+  return dict_get_value(tab->tp_vars, name, err);
+}
+
+Object tabpage_set_var(Tabpage tabpage, String name, Object value, Error *err)
+{
+  Object rv;
+  tabpage_T *tab = find_tab(tabpage, err);
+
+  if (!tab) {
+    return rv;
+  }
+
+  return dict_set_value(tab->tp_vars, name, value, err);
+}
+
+Window tabpage_get_window(Tabpage tabpage, Error *err)
+{
+  Window rv = 0;
+  tabpage_T *tab = find_tab(tabpage, err);
+
+  if (!tab) {
+    return rv;
+  }
+
+  if (tab == curtab) {
+    return vim_get_current_window();
+  } else {
+    tabpage_T *tp;
+    win_T *wp;
+    rv = 1;
+
+    FOR_ALL_TAB_WINDOWS(tp, wp) {
+      if (tp == tab && wp == tab->tp_curwin) {
+        return rv;
+      }
+      rv++;
+    }
+    // There should always be a current window for a tabpage
+    abort();
+  }
+}
+
+bool tabpage_is_valid(Tabpage tabpage)
+{
+  Error stub = {.set = false};
+  return find_tab(tabpage, &stub) != NULL;
+}
+