Feature Release 04

This release is dedicated to audio and video:

• Add Avisynth support for Linux and fix several bugs and crashes.
• Add BestSource video and audio source. This source, while several orders of magnitude slower than other sources, is essentially the only source that can absolutely guarantee being frame or sample accurate.
• Add VapourSynth video and audio source. Unlike Avisynth, VapourSynth is not shipped with the installer, so VapourSynth needs to be installed for the source to work.
• Add wangqr's Xaudio2 back-end and allow stereo playback with both this back-end, and DirectSound (PR'd by Shinon)

Apart from previewing Avisynth or VapourSynth scripts, these back-ends also allow for using further alternative sources to FFmpegSource, such as LSMASHSource or DGDecNV.

Other changes in this release include

• Add a Lua API function to check if there are unsaved changes. This will be useful for scripts like petzku's EncodeClip or my GitSigns script to make sure the user saved the file.
• Merge sepr0's Lua API function to read the current frame.
• Adjust default bounds for wxSpinCtrlDouble fields in Lua dialogs to fix dialog sizing. Note that this changes the behavior of Lua dialogs, but not in a way that contradicts existing documentation or specification.
• Fix folds not being loaded from a saved file if they start at the very first dialogue line
• Fix Aegisub crashing when trying to open a file from an invalid path. Internally, this is a fix for Avisynth, as it was caused by Avisynth not being unloaded properly after having been tried as a provider. Nevertheless, this seemingly unrelated crash has also been fixed with this.

MacOS build coming soon - BestSource refused to compile there.