Add nextVisibleWordEnd and prevVisibleWordEnd methods

[Galunid]

Add two methods that allow us to get the next/previous word from xpointer.

[Frenzie]

Just a quick superficial nitpick. Nothing else jumps out but I'll leave the rest to @poire-z

[poire-z]

Yeah, forget the tabs, stick to 4 spaces. There are still some functions with tabs, that we just haven't removed to not add unuseful commits.
And either keep the blank lines after the parameters checking and the one before the return parameters, or just drop the blank lines (for such little functions, you may as well remove these blank lines).

You may as well add now the prevVisibleWordStart and nextVisibleWordStart even if you don't use them yet.

[Frenzie]

This one stands out a bit. :-P

[poire-z]

If you can fix that last tab before {NULL, NULL}, we can merge this PR, so you can then bump base in your frontend PR.
(I described how to do that on the frontend side in koreader/koreader#4314 (comment) in case you're not familiar with submodules.)

[poire-z]

Looks like you need to fix (in your other PR) the 4 functions. For each of them:

     if (nodep.isNull())
         return 0;
-    nodep.nextVisibleWordEnd();
+    if (!nodep.nextVisibleWordEnd())
+        return 0;
     lua_pushstring(L, UnicodeToLocal(nodep.toString()).c_str());
     return 1;

Possibly only needed for the next* ones, but doing if for all is a good thing.
Because these nextVisibleWordEnd() return false when the move is not possible (while still moving the xpointer into a non-valid position - it becomes bad and corrupted when it return false, so it shouldn't be used for anything).

Noticable if you highlight some of the last text in a document, and move selection end ++ till you reach end of document. Next move will return false, but move the xpointer to the start of document, making moving back again impossible, highlighting all the document and koreader super slow.