Allow configuring script's screen edges from the KCM

Summary:
Modify the kwinscreenedges KCM to also list scripts which support screen
edge activation and read/write the appropriate value in the script's
config.

In order to only show relevant scripts an additional .desktop metadata
field is added.

Test Plan:
Opened KCM set a hot corner for minimize all.
Tested it
unset it, and set on another corner
Tested again

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: graesslin, plasma-devel, kwin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2972