Plasma 5.18 as LTS

[David Edmundson]

Following up on the previous threads:
 * We said we should ask distros when a next LTS suits them
 * 2 big distros have requested 5.18 (Feb 2020)
 * We had a chat at Akademy

Decisions:
 * Plasma 5.18 will be LTS
 * It will have to depend on Qt 5.12
 * Plasma 5.12 will then fall back to important patches only, releases
when needed.

Long term:
 * Our next-next LTS will be effectively at the same time as Plasma
6.0 depending on the final Qt5 and final frameworks 5.

David