Update WinGUI build script: allow SignTool.exe location to be specified and assume...

Update WinGUI build script: allow SignTool.exe location to be specified and assume x64 (#1376)

* WinGUI: fix `SignToolLocation` typos in build.xml

Previously, a mismatch between the condition (which checked for an empty `SignToolLocation`) and the action it took (by setting `SighToolLocation`) made it difficult to override the actual location of `SignTool.exe`. The bug was introduced in HandBrake/HandBrake@d375071be1158deb73ccb1d262310f15f231eca9

* WinGUI: Assume x64 platform in examples

Since 32-bit Windows is no longer supported (HandBrake/HandBrake@ecbd10efbdf286b9a5248fd0a870036cd4437360), the comment at the top of `build.xml` for WinGUI shouldn't need a platform property anymore. `build.xml` is already set to assume x64 as the default platform.

* WinGUI: minor typo fixes

Just a few aesthetic changes with no effect on functionality