Add BTU mod for the REVC top (#22)

* Remove V3 and V4 bearing tops based on REVA top

- The ball used in newer Ploopy trackballs doesn't fit in the REVA top -
  it's just slightly too big.
- I've designed an updated BTU mod for the REVC top, which should work
  fine with the older ball.

* Move README images to img/ subfolder

* Add blank REVC STEP and STL files

- These are the official Ploopy Top REVC, edited to remove the bearing
  sockets.
- The idea is that these can be used for any future mods to use
  different bearings.

* Add REVC versions of the BTU top STEP and STL files

* Rename v4 BTU mod FreeCAD file to match naming scheme

* Remove FreeCAD file for v4 BTU top

- It's not really relevant any more as it only works for the smaller
  ball

* Add REVC BTU top mod to the README

- Also explains that the v3 and v4 ones can still be found in the commit
  history

hardware/Mechanicals-BTU-Mod/README.md

@@ -11,17 +11,19 @@ This mod is a new 3D-printable top piece for the Ploopy which contains holes for
 Designer: [George Bryant](https://github.com/gbrnt) with help and bearing advice from [/u/d4baller](https://reddit.com/user/d4baller)
 
 ## Versions
-### v3
-This version was designed in Rhino and so isn't really editable for most people. It works well though.
-Print this version if you like seeing the exposed edge of the BTU.
+### REVC-BTU
+This version is the version you should use.
+It supports both the original ball and the new larger (45mm) ball.
 
-![A trackball with this mod v3 installed](example-v3.jpg)
+### v3 and v4
+These versions are too small to fit the new larger ball.
+They've been removed, but you can still find them in the commit history.
+v3 is a non-editable version that exposes the edge of one BTU, and v4 is an editable FreeCAD version.
+Here's what they look like:
 
-### v4
-This version was redesigned in FreeCAD and the various dimensions are now editable in the FreeCAD file.
-Choose this version if you want the BTU to be covered, or to be able to edit the file.
+![A trackball with this mod v3 installed](img/example-v3.jpg)
 
-![A trackball with the v4 mod installed](example-v4.jpg)
+![A trackball with the v4 mod installed](img/example-v4.jpg)
 
 ## Buying the BTUs
 The particular BTU this mod is designed for is the Bosch-Rexroth R053010810, or KU-B8-OFK. A set of three is about 20-30 USD, depending on where in the world you are and where you buy them from. Here are some sources we've found so far:
@@ -56,4 +58,4 @@ Recommendations for installing these are in the [Ploopy trackball assembly guide
 ### Modifying secondary buttons
 There is a small tab on the secondary buttons which collides with the closest BTU. It needs to be sanded, filed or cut off. It is indicated in red in the image below:
 
-![Ploopy secondary buttons, with the tab on the end highlighted in red](trim-tab.jpg)
+![Ploopy secondary buttons, with the tab on the end highlighted in red](img/trim-tab.jpg)