kcmodule: Change inactivity labels

The label for configuring the action to take when inactive (and the duration required to count as such) currently reads as follows: "After a period of inactivity: [Sleep] [After 15 minutes]"

This is somewhat confusing, as it reads like a sentence but does not quite make sense as one. It implies that there are two durations, one of them being of unspecified length, and the configurable duration only begins to count after this. It could even be read in such a way that if the user was inactive for a period of time, it will sleep 15 minutes later even if the user interacts with the computer again during these 15 minutes.

This change modifies the wording to read: "When inactive: [Sleep] [After 15 minutes]" This rules out the incorrect reading, is much shorter, and makes the text more similar to that of the surrounding options, which all use a "When x, do y" form. Some related text is also updated to match this.

This requires changing the keyboard accelerator from Alt+F to Alt+I, as there is no F in the new text.

Note: The new keyboard accerelerator does not actually work; same problem as the old one: https://bugs.kde.org/show_bug.cgi?id=489799

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