app: presentation tweaks and improvements

[Ramen2X]

This PR includes some general presentation tweaks and improvements:

• Sets the global application title to "SI Editor" and reuses it for the window title (previously, only the window title was set to "SI Editor").
• Appends the currently opened Interleaf file name to the window title.
• Adds an asterisk indicator to the filename in the title if the file has pending unsaved changes.
• Properly clears/initializes the screen when making a new file (previously, the media panel elements would remain).

[itsmattkc]

Good additions, though Qt has a more automated way of adding * to the title that's probably preferable:

Call setWindowTitle() with a [*] somewhere in the string, and then call setWindowModified() and Qt will automatically add/remove the asterisk. This is also useful because at some point we can check windowModified() to confirm/prevent closing an unsaved file.

[Ramen2X]

On the topic of automating things with Qt, I just noticed that we don't necessarily have to manually handle trimming directories if we make use of QFileInfo and use its methods to get the clean filename, like it's used here:

SIEdit/app/mainwindow.cpp

Line 383 in 8e25f45

v->SetSubtitle(QFileInfo(s).fileName());

This would probably be more "correct", but even if we were to do this, we'd still have to manually trim the embedded media paths to get the Object filenames in the SI for the extraction functionality. So unless there were some way to encapsulate those in a QFileInfo object as well, I wonder if keeping the custom function makes sense. We could also go back to inlining the trimming logic in the extraction function only, like it is in master.