Basic completion support

include/ui/bar.h

@@ -8,7 +8,7 @@
 struct Bar {
   Coord screen;
   std::string command = "";
-  int cursor = 0;
+  uint32_t cursor = 0;
 
   Bar(Coord screen) : screen(screen) {}
   void render();

include/ui/completionbox.h

@@ -6,6 +6,7 @@
 #include "utils/utils.h"
 
 struct CompletionBox {
+  std::shared_mutex mtx;
   struct CompletionSession *session;
   bool hidden = true;
   std::vector<ScreenCell> cells;