Add index on type to filemarkup table

[garybuhrmaster]

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Fixes #873

Lightly tested (compiles, generates the index, updated the dbver, explain on a select shows possible use of key (rather than using where)).

Note for users: the mysql/mariadb optimizer may, or may not, use any table index, especially if one has not run an "analyze" on the impacted table to update the statistics used for determining the query plan (one may need to run the optimize_mythdb.pl script (or equivalent) to see any performance improvements).

As a schema change, per the project's standards, this can only be applied to the development branch and not an existing release.