Plasma 5.18 as LTS

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