cache MethodInfo (use reflection only once)
[RFC] invalidate SystemColors / SystemResources on a new thread

- This gets rid of the small UI hickup when changing theme/accents (noticeable if you have A LOT of complex ui elements, like charts)
We have TPL now...
use tasks depending on property
rename property to make some sense.
Merge pull request #948 from remcoros/cache_methodinfos

[RFC] Cache methodinfos & Invalidate system caches on a seperate thread