Hide the modules list when leaving edit mode

Otherwise, it's still possible to add a module.
Assign a few keybindings to the top menu's buttons
Don't close the module list after adding a module

Usually, the user adds multiple modules, and can then close the list
theirself.
Expand the module tree view by default

As long as the number is this low, it's annoying to have to open all
categories by hand.
When removing the selected module, set it to NULL

Otherwise, when pressing "Remove" again, we will try to remove an already
deleted module.
Merge pull request #3 from blinry/improved-module-handling

Improved module handling