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