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/popupmnu.h

@@ -0,0 +1,19 @@
+#ifndef NEOVIM_POPUPMNU_H
+#define NEOVIM_POPUPMNU_H
+
+/// Used for popup menu items.
+typedef struct {
+  char_u *pum_text;        // main menu text
+  char_u *pum_kind;        // extra kind text (may be truncated)
+  char_u *pum_extra;       // extra menu text (may be truncated)
+  char_u *pum_info;        // extra info
+} pumitem_T;
+
+void pum_display(pumitem_T *array, int size, int selected);
+void pum_redraw(void);
+void pum_undisplay(void);
+void pum_clear(void);
+int pum_visible(void);
+int pum_get_height(void);
+
+#endif  // NEOVIM_POPUPMNU_H