Add (experimental) support for topbottom as well as sidebyside 3d mode

Fix crash on using cursor keys in client menu without selected server
Add support for non fixed size tabviews

Fix client not showing hearts and bubbles on connecting to old server
Fix server not remembering hud flags correctly

Add DPI support for statbar
Move heart+bubble bar to Lua HUD
Add statbar size (based upon an idea by blue42u)
Add support for customizing breath and statbar

Minor coding style fixes

This makes a number of changes:
  * Remove the dependency on marshal by using string.dump and loadstring.
  * Use lua_tolstring rather than having Lua functions pass string lengths to C++.
  * Move lua_api/l_async_events.* to cpp_api/s_async.*, where it belongs.
  * Make AsyncWorkerThread a child of ScriptApiBase, this removes some duplicate functionality.
  * Don't wait for async threads to shut down.  (Is this safe?  Might result in corruption if the thread is writing to a file.)
  * Pop more unused items from the stack
  * Code style fixes
  * Other misc changes

Add support for (configurable) multiline hotbar
Improved screensize handling
Add userdefined gui scale by BlockMen

This reverts commit 5b518ed2.

This caused duplicate tracebacks and tracebacks when unwanted.
It also ignored the level argument to error() and didn't pass the message (or level) to debug.traceback().
Use xpcall() or lua_pcall()'s errorhandler argument instead.