Font families are now cached. There probably should be a mechanism to
refresh the cache, but we don't currently have a "signal" to notify us
when a font is installed. Presumably, this could be something that
fontdb would eventually add, rather than us building our own.
Closes#48
After trying to run Gooey again on my Mac for the first time in a few
weeks, I found that I ran into the Condvar issue again. Rather than
pasting AssertUnwindSafe in those files, I've both reported the
discrepency in unwind safety (rust-lang/rust#118009) and moved the
workaround into a type that only uses AssertUnwindsafe when compiling
for Apple.