bluViewport: Add ImageWidth/Height properties
Reimplemented hover rectangle and painting!

This is now an incredibly basic, but FULLY OPERATIONAL BATTLE STATION!
:) You can import a ROM, change blocks in the level and export a
playable ROM.

* bluViewport now sends a MouseMove event whenever it scrolls since the
image point under the cursor (ImageX/Y) will have changed. This is
incomplete at the moment as I need to fetch the mouse button / key state
to go with it (this would mean that if the user scrolls whilst holding
shift / ctrl keys, we can keep constant, uninterrupted track of it)
* bluViewport now repaints _after_ sending events, so that your code
does not need to call `Refresh`, this minimises repaints
* Removed a load of code and variables from frmLevel that were
previously used for the old scrollbar controls
* Reimplemented the hover rectangle using the bluViewport `Paint` event
* Reimplemented block painting (infinitely simpler now that we have
bluViewport -- the whole point of writing it)