Behavior when opening files on macOS and Windows #391

As far as I can tell, there are 4 (essentially 3 on macOS) ways to open a file in Aegisub. They work inconsistently on the two OSes:

1. Dropping files on the application icon (does anyone do that?):

• On macOS, Aegisub attempts to combine the files into a single project (e.g. subs + video) and opens it in an existing window. If there are unsaved changes, they are silently discarded.
• On Windows, same as on macOS but in a new window.

2. Double-clicking in Finder/Explorer:

• On macOS, same as the first due to the way the OS works, except only subs are likely to be opened with Aegisub, so only one file ends up being opened.
• On Windows, each file opens in a new window.

3. "Open subtitles file" dialog:

• On macOS, the file opens in a new window.
• On Windows, the file opens in the same window. If there are unsaved changes, Aegisub displays a prompt.

4. Dropping files on an Aegisub window:

• On macOS, same as the first two.
• On Windows, same as on macOS (including silently discarding unsaved changes).

I propose cleaning up this mess. In particular, at least the following should be true:

• Unsaved changes are never silently discarded
• If the file isn't explicitly opened in a specific window, it opens in a new window

What do you think?