Human readable duration search

Addresses this (https://bugs.launchpad.net/mixxx/+bug/1261493) wishlist bug

MIXXX can only search duration in seconds.
Searching tracks with duration keyword is made more natural. User can
search now in following formats

2:30/2:30m
2:30:45/2:30:45h
2m30s/2m30
2h30m45s/2h30m45

In user manual we should encourage our users to use top two formats.