Instead of connecting to the global `exit_app` signal, pass the signals
through their respective widgets. Because signals are queued by
default, this ensures that slots are executed in the order they are
connected. Makes it cleaner to do cleanup procedures where they should
make sense, unlike the global signal, where it has to be inferred by
the widget instantiating order.
* Add RareStyle qrc with the icons needed for comboboxes and spinboxes
* Add images as svg
* Refactor `styles` to `resources` since it contains more things now.
* Add stylesheet folders, each folder must contain a `stylesheet.qss` file.
* Don't use QResources, instead add an identifier in the qss url() which can
replaced later with the full path.
* Update RareStyle with SVG images for better quality.
