This PR leverages earlier work landed previously in #174 and adds a basic user interface for loading/viewing and exporting debug logs for both Gridsync itself and its underlying Tahoe-LAFS processes (accessible via the system tray menu under "Help" -> "Export Debug Information..."). In addition to exporting logs to either a file or the system clipboard(s), this feature also provides the functionality (and which is "on" by default) to apply ad-hoc filters to the loaded/exported log buffers, stripping out most -- but probably not all -- potentially sensitive or personally-identifiable information (including known Tahoe-LAFS fURLs and capabilities, folder and grid names, magic-folder member names, and most directories/paths).

For now, users who wish to adjust or disable the in-memory log buffer(s) can do so by editing the log_maxlen value in the [debug] section of config.txt (with a value of 0 disabling the buffers completely). Friendlier user-facing controls for adjusting the behavior of this feature within the application will be added at a later date.