Allow other units in speed labels

[yezhiyi9670]

Currently the speed can only be specified in the form of quarter-notes-per-minute (this is also wrongly referred to as BPM). This is sometimes inconvenient, for example in the 6/8 tempo, a beat is not equal to a quarter note, but a semi-quarter note, and speed is usually specified in semi-quarter-notes-per-minute or dotted-quarter-notes-per-minute.

▲ In this case, the user have to translate this speed into 106.5 qpm and then type it into OpenUTAU.

It would be helpful to have a speed unit dropdown in the speed dialog. Moreover, the selected speed unit should be remembered in the project file so the user does not have to select it every time.