[Bf-codereview] Fix for #36226, Select Linked works not in touch with Prefs. (issue 13244048)

[lukas.toenne at gmail.com]

Made some UI improvements:
* Buttons get a new flag for drawing unset properties. This can be
activated by passing an according flag to uiItem functions. It is only
used in the keymap user prefs, no need to show this info in other places
so far.

* Tooltips get an additional *UNSET* string attached for unset
properties. This also happens only in the keymap user prefs.

https://codereview.appspot.com/13244048/