Add new branch label format option for bare percent

[zygoballus]

I'm surprised that FigTree doesn't offer an option for bare percent values for bootstrap branch labels, i.e. percent without the percent sign. This seems like a very common format used in the literature. For example "92" instead of "92%". This is especially helpful for situations in which you have very little space for the values.

BTW, I'm loving the new 1.4.5 pre-release! It has been working flawlessly for me in MacOS Big Sur (MacOS 11).