Add new "Tools" button above System Monitor's process list

Summary:
This adds a new "Tools" button to the libksysguard widget which opens a menu that contains tools that help with system diagnostics.

• Patch migrated from ReviewBoard https://git.reviewboard.kde.org/r/128854/

Screenshots:

Original (English):

Localized (German):

Some tools not installed / old menu version:

Test Plan:

• Verify that each action in the menu works.
• Verify that if the global keyboard shortcut for Kill Window is changed, that it is updated accordingly in the menu item.

Reviewers: Plasma, colomar, kossebau, broulik, mart, hein, rkflx

Reviewed By: rkflx

Subscribers: apol, anthonyfieroni, andreaska, rkflx, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10297