REF: time string parsing

Parse directly with a qregexp. Simplify the code. This also changes the
format of time strings that are supported. To search for a track of
exactly 90 seconds the following terms are equivalent.

90
90s
1m30
1m30s
1:30

searching for 62 seconds can be entered as

62
62s
1m2
1m2s
1m02s
1:2