This allows us to request any specific editor to close itself. Earlier,
this could only be done for the currently active editor, so trying to
close inactive tabs would not work properly.
Now, when trying to close the application, there is a separate prompt
for each open tab with unsaved changes. Each tab is closed after it is
handled appropriately (assuming the user didn't Cancel), this makes it
so that the message box is always asking about the currently active tab,
allowing the user to see that the image contains.
If at any point the user presses "Cancel", all remaining tabs are kept
open.
After transitioning to FileSystemAccessServer, some of the methods
in `MainWidget` and `ProjectLoader` for opening files directly with
a filename as opposed to with a file descriptor are unused. This
commit removes them.
We can now drag-and-drop files onto PixelPaint to be able to open
them. Each dropped file opens in a separate editor (which is the
default behavior of Photoshop).
Previously, all the UI setup was done in `main.cpp`, with a whole
bunch of lambdas,, and actions etc just being stored in the main
function. This is probably an artifact from back when it was first
created.
Most other applications now have a "MainWidget" class of some sort
which handles setting up all the UI/menubars, etc. More importantly,,
it also lets us handle application-wide events which we were
previously not able to do directly, since the main widget was just
a default GUI::Widget.
This patch moves all the core functionality of the PixelPaint
application into PixelPaint::MainWidget, which is then instantiated
by the main function. There is likely some more refactoring that
would help, but this commit is big enough as it is doing mostly
a direct port.
