MNE_BROWSE_RAW_QT: New Features

- added overlap-add filtering: select channels -> right-click -> filter all/filter selected only
- added multithreading -> reloading and filtering operations are performed in background threads
- FilterOperators (e.g. FIR filters) can be generated during runtime using Parks McClellan algorithm (matlab: see firpm function)

Would appreciate if you could check that everything runs fine without any crashes. Thanks guys!