Add support for right-aligned roomtext

This is mostly so people making levels in an RTL language have a more
pleasant and logical experience. If roomtext is placed in a level set
to RTL, it will get p1=1, which makes that roomtext right-aligned.
Because, imagine for English you click to place roomtext, and the text
runs left of where you clicked, which wouldn't be logical.

Since it's an entity-bound property, switching RTL on and off either in
the editor or via a script does not affect existing entities.

desktop_version/src/Map.h

@@ -16,6 +16,7 @@ struct Roomtext
 {
     int x, y;
     const char* text;
+    bool rtl;
 };
 
 enum RoomnameType