Fix to initial lag in drag in large projects and also issue 667.

Removing the onchange handler in lexical_variable_get and lexical_variable_set
eliminates the remaining delay in starting a block drag in large projects.
It also fixes issue 667 in which copying variable setters and getters out
of scope changed the current names to the empty string. Now the names remain,
but the getter/setter block has a red error triangle.

This does *not* improve many other nasty lags involving large projects:
* loading large projects is numbingly slow
* mouse-up on a moved block is very slow because of an unexpectedly large
  number of render calls
* displaying a drop-down list for event-handler component or variable
  is unexpectedly slow.

Change-Id: If06a3011d5b31fc5308fc33e99c2a0adb2fdb160