allow me to view multiple source files simultaneously

[Adam Dingle]

Currently in Valide I can view only one source file at a time. This is an
unfortunate restriction. Here are two ways we could let the user view
multiple files simultaneously:

1. Allow the user to open a file in a separate window. (The TODO list
already contains a task 'Context menu: Detach tab in new window';
presumably this is the same thing.)

2. Allow the user to split the Valide window into two panes side by side;
the user could drag tabs from one pane to the other.

[Nicolas Joseph (gege2061)]

Yes, I choised the first idea.

[Adam Dingle (adam-yorba)]

I think the second approach might be easier to implement. With the first approach,
do both windows show all the Valide menus, or does the extra window just show some of
them? Can tabs be moved from window to window? Can both windows show the side panel
or the bottom panel including build output? The second approach avoids these
questions since there's just a single window.