[Lazarus] Clipboard method needed to clear cache

[AlexeyT via Lazarus]

LCL uses clipboard cache to support all clp formats on all OS, even if
OS don't support some.

In *CudaLister plugin for Total Commander* [based on CudaText parts], I
need method to clear this cache. Clipboard.Clear is not ok [i need
contents of OS clp]. Method needed to solve this issue:

https://github.com/Alexey-T/CudaLister/issues/27 (english text also). I
will call it before reading Clipboard.AsText.