Fix check for _NET_WM_STATE_SKIP_TASKBAR

The property was never checked in reality.
Request properties in one go

Two calls are not needed. Based on review, request only 12 properties.
Also, added a missing XFree.
Merge pull request #33 from mikkoharju/skip-taskbar

Fix check for _NET_WM_STATE_SKIP_TASKBAR
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Add a possibility to remove all user removable notifications
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Remove StatusBar component. Fixes NEMO#340.

Nemo's homescreen no longer uses this, and lipstick itself was always rather the
wrong place to put this.
Expose notification item count in the Notification class
Remove unused stub
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Expose notification priority in the Notification class
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Mention x-nemo-priority in the documentation
Allow customization of NotificationListModel behavior in derived classes
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Expose notification category in the Notification class
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Export HomeWindowMonitor to UX implementations
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Disable notification previews when the touch screen is locked and the display is off