Move to the Network main category
Ease development by prepending to sys.path instead of appending

When sftpman-gtk is already installed as a package on the system,
and one tries to run `sftpman_gtk/launcher.py` during development,
the global files are going to be loaded, not the ones we want.

Appending to sys.path doesn't help, because the global packages path appears
first either way. It only works if there is no globally installed package.

Prepending fixes it for all cases.
Use scrolled window to prevent unlimited window height growth

When there are lots of sftp systems to show, the list would expand
and the whole window would grow in height indefinitely, not even
constrained by the size of the screen.

Using a scrolled window allows us to have a fixed size viewport
for the list and scrollbars when necessary.
Move version and other related information to the package init file
Add About dialog
Release 0.4.0