gnome-tweak-tool: Minor additions

This update fixes the following issues for gnome-tweak-tool:
- bnc#770048: Add gnome-tweak-tool-pwr-button.patch: Allow to configure what action should be performed when pressing the power button (acpi)
- Drop xz BuildRequires as it now comes for free in the build system.

Fixed bugs
bnc#770048
make power button action configurable
Selected Binaries
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