Improve package dependency graph integration

[fcollonval]

What do you think of this mock-up?

The idea is to add an icon in the toolbar above the packages list (it will be absent if mamba is not available - or disabled with a title advertising mamba 😉 ).
On click on the button, this would open the dependency panel (not sure if on the right of the package list is a good place or if it needs to be a separated panel to allow positioning and resize by the user).

Then selecting a package will result in the display of its dependencies graph. If not package selected, a help message such Select a package to display its dependency graph.

This means also to change the current behavior as onClick on a row changes the status of the package. This should then be restricted to the status icon probably.

Originally posted by @fcollonval in #102 (comment)