Add Meta+Space as a default shortcut to activate KRunner

Summary:
There are two proposed justifications for this addition:
- In general, VDG would like to move towards consistently using the Meta key for global shortcuts
- Switching macOS users will be able to activate KRunner with the exact same keyboard shortcut they used to activate Spotlight

Reviewers: #vdg, #plasma, ndavis, GB_2

Reviewed By: #vdg, ndavis, GB_2

Subscribers: GB_2, romangg, ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24033