UI: Batch remux and drag/drop support on remux dialog

This changes the remux dialog to support a collection of input/output
pairs, presented as a QTableView.  Standard Qt icons are used to
indicate the state of each entry during a remux operation.  Drag/drop
support is added to populate the list quickly.  Both Dark and Rachni
themes are updated to make QTableView look reasonable.

Relevant text is added in the localization files.

Closes obsproject/obs-studio#1153