Add method for getting info about a package

Returns sub-packages and functions in that package
Add method to get location information about a function
WIP for picking additional files to load
Opening a new file window is minimally working
Fix some issues when a new file tab is opened

It is now resized properly

The requested sub is now scrolled into view
Limit the width of file window tabs
File windows/tabs are now closable
Fix rendering for the file window close button
Filewindows now update their breakpoints
Don't show subroutines that aren't debuggable in the file picker

These are probably C functions with Perl bindings
Allow loading functions from package 'main'