garcon: Escape '%' when building command from URL desktop files

Since Garcon reduces the case of URL desktop files to the case of a
classic file with an Exec key of value `exo-open 'URL'`, and there is no
reliable way to distinguish the two cases from the outside when invoking
`garcon_menu_item_get_command()`, Garcon must escape the '%' characters
in the URL as if they appeared in the value of an Exec key.

Fixes: #31