Rename Set param consistently to v
Add ByteProperty
Add Uint64Property
Optimized paint request and throttling algorithm

Previously a repaint was requested through method Paint() and was
synchronous. Frame throttling was computed using time.Now(), which is a
relatively expensive call. It also led to a bug were a dropped frame
would not be redrawn if repaint wasn't explicitly requested after the
frame had been dropped.

Now tuikit.Init() returns a sending channel. The client writes to the
channel to request a repaint. This decouples painting from application
logic and allows for a more fine-grained throttling algorithm. As a
side-effect the bug mentioned earlier is fixed and there isn't any call
to time.Now() anymore.

The examples were also adjusted to use the new channel.
Add tktop example
Build tktop, minor changes to build logic and README