Create QC entries directly from an *.ass subtitle file

Create QC entries directly from an *.ass subtitle file

Create a new ftdetect and ftplugin for *.ass subtitle files so we can
define a mapping for these files.

Create a mapping in *.ass files to easily add lines of dialogue to a QC
file. This mapping expects the current tab to contain 2 windows, one
with the subtitle file and the other with the QC file. Invoking the
mapping from the subtitle file adds the line of dialogue on the current
line as the last entry in the QC document.

In order to be able to call the `NewEntry()` function from the new *.ass
ftplugin, I needed to move the function to 'autoload'. The `sub_qc`
ftplugin's mappings are updated to the function's new location.