Add support for rit./accel.

[Canaanabolaanan]

Rit./Accel. measures operate at a constant pace for the duration of measures between the current and ending tempo markers. The pace is fixed regardless of the tempos before or after them, or whether they are higher/lower than the rit/accel constant tempo pace.

If the rit/accel markers occur within a measure, they'll play at the aforementioned constant tempo, but the measure itself occupies the space of the measure as set by the tempo marker before it.

If the rit/accel markers are placed on the barline instead, there is no extra empty space at the end of the measure.

The Rit. tempo is somewhere above 300BPM (the limit for PTE1.7). The Accel. tempo is at about 250.

In 1.7, a rit. and an accel. perform identically. The only thing that truly matters is the starting/ending tempo markers.
For instance, a
(t=120 -accel- t=180) will play identically to a
(t=120 -rit- t=180.)

Test files:
https://onedrive.live.com/redir?resid=BF1A7BA5AAA1B4E4!157&authkey=!AOxEDsTIUlHlVk4&ithint=folder%2c

[cameronwhite]

We can currently create alterations of pace and import them from v1.7 files, so we just need to add MIDI support.