Do var replacement in Var event, too.

Testing done:

Ran script with <var var="a" set="{@foo}"/>
Add git revision to summary.html

Testing done:

Ran VDDReporter and checked output.
Rework Event.replaceString

The var finding regex should now be a bit more efficient and the code is
definitely more readable.

Testing done:

VDD regression suite
Replace vars in assert/assertnot

Testing done:

VDD regression suite and extra script with vars in assert.
Rearrange HtmlEvent.click logic

Testing done:

VDD regression suite.