Fix tests compilation
Clean up OptionValue

Make the vtables less absurdly huge (knocks 100KB off aegisub32.exe),
eliminate some copies of the values when constructing the options, and
use an enum class for the value type.
Define OptionValueMap with using rather than inheritence
Remove redundant option not found exception type
Add a default value for GTEST_ROOT
Actually include the dependency files in the tests Makefile
Remove array indexing from UnknownElement
Sort the audio display markers

This was accidentally removed in 5d3bc0d. Closes #1731.
Update row numbers after an order-changing commit