applet: Raise maximum volume: Rework anchors into QQC2 sizing model

Controls are more flexible and clickable with paddings instead of
anchors. Now the checkbox respects (almost) Fitt's law by covering most
of the footer. Covering the whole area deems unfeasible, because of how
PlasmoidHeading's theming works with its own padding being applied.
Meanwhile, the checkbox should be alright, and its content is
vertically centered out-of-the-box.
2 jobs for !94 with work/ratijas/layout in 50 seconds (queued for 15 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed freebsd_qt515 #153596
FreeBSD

00:00:21

passed suse_tumbleweed_qt515 #153595
Linux

00:00:38