MainFrm: remove seek-on-keyframe legacy hack

It breaks accurate keyframe seeking in LAV
Add seek-on-keyframe rounding method selection
AppSettings: seek to nearest keyframe by default
MainFrm: Get the source filter from the graph immediately after opening the main file.

Changed all code that depends on the source filter to take advantage of it.
Fixes issues with the wrong source filter being used when multiple source filters are on the graph.