Change version to match KDE Applications (17.11.70)

Summary:
Previously krfb had version set to 5.0. This is misleading.
Now, do the same as other standard KDE apps, like ark or dolphin: use
KDE_APPLICATIONS_VERSION_MAJOR, KDE_APPLICATIONS_VERSION_MINOR, etc.
This page: https://www.kde.org/applications/system/krfb has a link to report a bug: https://bugs.kde.org/enter_bug.cgi?format=guided&product=krfb
Versions in bugzilla have 17.xx.xx, so "About..." window should match.

cmake: Set version in project(); use ecm_setup_version() call

Inspired by commit in Ark: cffecfbfffd22bc0bbe451ed8d74f3b6da113c6f

Test Plan: Compile, run. Make sure that Help -> About Desktop Sharing displays the correct version.

Reviewers: aacid, ltoscano

Tags: #kde_applications

Differential Revision: https://phabricator.kde.org/D7658