Code and UI cleanups + new "noise seed" feature

- Clean up the ntsc.py code - This includes various typing fixes, cleanup of imports, and removal of unused code like `XorWowRandom`. - The `LowpassFilter` class and `lowpassFilters` function have been replaced with a single `lowpassFilter` function. As far as I can tell, the `lowpassFilters` function just created the same lowpass filter 3 times. - UI cleanups and improvements - Some UI elements have been moved to the sections they're logically related to. Namely, the "Pro mode" checkbox has been moved above the controls section, since it enables more controls; and the "Live preview" checkbox has been moved down onto the row of render settings. - The slider settings on the left are now connected to SpinBoxes. It looks like someone attempted to implement this before, but `DoubleSlider` was returning the wrong value. That's been fixed, so now we can use SpinBoxes! - The slider settings are now laid out in a `QFormLayout`, which makes things much tidier. In addition, they now scroll if there are too many on screen at once. - The "Color bleed before" checkbox has been moved so it no longer overlaps with the "Cut 2% black line" checkbox. - The random-preset seed generator thing now requires you to press a button to override your current settings. - The file picker now filters by only the files that the program actually supports. There's an "All files" option still, if for any reason the user wants to view all the files in the picker (e.g. to see if the file they wanted to load is in the right place, but is just not supported). - The "VHS emulating" checkbox is now grouped up with all of the options it enables. This makes it clearer which options have no effect when "VHS emulating" is disabled. - I've added a new "noise seed" setting, which allows you to set a seed for all video effects that use random number generation (e.g. video noise, edge wave). This means that a given frame should always render identically given the same input settings. - I've implemented the "VHS speed" control, so users can now edit the setting themselves. - A couple bugfixes: - "Cut 2% black line" used to overwrite the input image, meaning it couldn't be removed. That's now fixed. - The VHS head switching effect now depends only on the frame number, meaning it should render more consistently.