CTRL+, and CTRL+. to modify platv

`platv` is a room property that controls platform speed, and it has
always worked (other than some weird storage issues due to a bug).
However, the editor has no way to edit it currently, so people had to
resort to editing the level file by hand, or with a third-party tool.
This commit simply adds an easy way to modify platform speed.

Author: AllyTally

desktop_version/lang/en/strings.xml

@@ -630,6 +630,7 @@
     <string english="Now using {area} Tileset" translation="" explanation="level editor, user changed the tileset of the room to {area} (like Ship, Lab, etc)" max="38*3"/>
     <string english="Tileset Colour Changed" translation="" explanation="level editor, user changed the tileset colour/variant of the room" max="38*3"/>
     <string english="Enemy Type Changed" translation="" explanation="level editor, user changed enemy appearance for the room" max="38*3"/>
+    <string english="Platform speed is now {speed}" translation="" explanation="level editor, user changed speed of platforms for the room" max="38*3"/>
     <string english="Reloaded resources" translation="" explanation="level editor, reloaded graphics assets/resources, music and sound effects" max="38*3"/>
     <string english="ERROR: Invalid format" translation="" explanation="user was supposed to enter something like `12,12`, but entered `as@df`" max="38*3"/>
     <string english="Loaded map: {filename}.vvvvvv" translation="" explanation="successfully loaded level file" max="38*3"/>

desktop_version/src/Editor.cpp

@@ -2771,17 +2771,46 @@ static void handle_draw_input()
         ed.x_modifier = key.keymap[SDLK_x];
         ed.z_modifier = key.keymap[SDLK_z];
 
+        const int room = ed.levx + ed.levy * cl.maxwidth;
+        const int plat_speed = cl.roomproperties[room].platv;
+
         if (key.keymap[SDLK_COMMA])
         {
-            ed.current_tool = (EditorTools) POS_MOD(ed.current_tool - 1, NUM_EditorTools);
+            if (key.keymap[SDLK_LCTRL] || key.keymap[SDLK_RCTRL])
+            {
+                cl.roomproperties[room].platv = plat_speed - 1;
+            }
+            else
+            {
+                ed.current_tool = (EditorTools) POS_MOD(ed.current_tool - 1, NUM_EditorTools);
+            }
             ed.keydelay = 6;
         }
         else if (key.keymap[SDLK_PERIOD])
         {
-            ed.current_tool = (EditorTools) POS_MOD(ed.current_tool + 1, NUM_EditorTools);
+            if (key.keymap[SDLK_LCTRL] || key.keymap[SDLK_RCTRL])
+            {
+                cl.roomproperties[room].platv = plat_speed + 1;
+            }
+            else
+            {
+                ed.current_tool = (EditorTools) POS_MOD(ed.current_tool + 1, NUM_EditorTools);
+            }
             ed.keydelay = 6;
         }
 
+        if (plat_speed != cl.roomproperties[room].platv)
+        {
+            char buffer[3 * SCREEN_WIDTH_CHARS + 1];
+            vformat_buf(
+                buffer, sizeof(buffer),
+                loc::gettext("Platform speed is now {speed}"),
+                "speed:int",
+                cl.roomproperties[room].platv
+            );
+            ed.show_note(buffer);
+        }
+
         if (key.keymap[SDLK_SPACE])
         {
             ed.toolbox_open = !ed.toolbox_open;