Fix indentation.
Fetch mtp device information (cached) in a task.

The method to open a device in cached mode is synchronous and takes
painstakingly long resulting in the UI being frozen. So this now
runs in a GTask. Works only with the latest version of glib.

Things to do:
* Make the icons unclickable during the operation.
* Provide a cancel button.
Remove stray comments.
Add functions to freeze and thaw the devicelistview.
Remove unused code.
Changes to opening of a mtp device.

This patch contains the following changes:
* Open device in cached mode.
* Add support for storage device enumeration.