This fixes the QAbstractItemModel invalid index warning.

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

This adds close buttons to remux dialog, output timer dialog, and
advanced audio properties dialog.  I also did a small refactor of the
remux dialog so the buttons were consistent with other dialogs.

Closes jp9000/obs-studio#876

This is to prevent confusion both when prefixing commits and when
reading the directory structure for the first time.