Made many little changes across the board

CustomSlider: setValueNoSignal method blocks signals while setting the value, this way we can consolidate and look for ValueChanged signals (without the NoSignal method, basic video-level updates will also fire the ValueChanged event resulting in the bar moving itself)
MpvHandler: Observing volume and path now to fire VolumeChanged and FileChanged events--the path has the full system path to the file that is being played (may change to media-info which will have url, path, or whatever depending on what it is), volume so that changing volume via mpv-input bindings propigates outwards to our gui.
mainwindow: Seek method for consolidation of seekBar's events via a connect statement and implementation of the setValueNoSignal method. Also Changed seek time to 10--mpv's default value.