Filtering tracks in library by length should handle abbreviations like lastplayed

[residuum]

Before posting

Please follow the steps below and check the boxes with [x] once you did the step.

• [x] I checked the issue tracker for similar issues
• [x] I checked the changelog if the issue is already resolved
• [x] I tried the latest Clementine build from here

System information

Please provide information about your system and the version of Clementine used.

• Operating System: Debian testing/unstable
• Clementine version: 1.4 rc2

Expected behaviour / actual behaviour

When filtering tracks in the library, it is possible to search for tracks played within a specified time with the syntax lastplayed:<1h30m. It is also possible to filter tracks by length, but not with the abbreviations for minutes, hours, seconds etc. So one must search for nanoseconds now, e.g. length:<60000000000 returns tracks shorter than 1 minute in the library, but length:<1m would be nice for that.

Currently the abbreviations are only filtered in date columns https://github.com/clementine-player/Clementine/blob/4ab6f35ec0e0083bc3596f92f39d0a98df62052c/src/library/libraryquery.cpp#L110