Add main category and icon to khal.desktop

[renanglr]

This pull request adds the "Office" category and the "x-office-calendar" icon to the khal.desktop file.

• Adding a category fixes the entry being placed in the "Lost and Found" category in KDE Plasma, maybe other desktops?
• Adding an icon makes it look good in the menus :-) I've chosen an icon that is available in most icon themes. It is an icon for a file type instead of an application, but it should look good enough.

[WhyNotHugo]

x-office-calendar is a well-known icon as per the Icon Naming Specification 👍