better start menu integration: provide tools, on-off via magic key, start if needed

[Antoine Martin]

The new feature in 0.11.3 to wrap regular commands via winswitch_command_wrapper is very neat (and fast too!), but for more people to use it we need to make it easier to setup: provide a tool to make it easier to convert a bunch of .desktop files to use/not-use the wrapper.

It needs to:

• be able to run as root to change system wide settings
• run as user and save to user specific shortcuts
• take the blacklists into account so we don't convert software that does not work (see #132)

The ​freedesktop menu specification seems to allow this sort of thing.

Also maybe:

• detect if the desktop files have been modified (ie: by OS upgrades) and re-apply the changes