Files/Folder selector for files in SNS

Generally all places expecting file paths from user inputs should be a file selector (also should allow users to input file path directly), including:

User input tab:
Output Folder

Instrument input:
Mask file
Dark file
Wavelength

As a matter of fact, all places that expect a run number, should be able to take a file containing the result from that run number. The run number input is originally just for the convenience of the users, the script of drtSANS will automatically locate the data file associated with that run number by first looking the local folder, then the designated data drive. But, again, let's skip that for now.