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):
{F5877637}

Localized (German):
{F5877629}

Some tools not installed / old menu version:
{F5712736}

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