Pause animation when elements are invisible

When `BusyIndicator` elements and the custom spinner in the sync
notification bar are not visible, their animations are now stopped.
Otherwise the GUI is constantly redrawn, causing additional CPU
consumption.
12 jobs for 201-avoid-constant-repaints-of-the-ui in 13 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Images
manual #66368361
allowed to fail manual
build_image_android
manual #66368358
allowed to fail manual
build_image_ubuntu
manual #66368359
allowed to fail manual
build_image_win32
manual #66368360
allowed to fail manual
build_image_win64
 
  Build
passed #66368368
android_armv7

00:11:16

passed #66368369
android_x86

00:11:39

manual #66368367
macos allowed to fail manual
macos
passed #66368362
ubuntu_appimage

00:13:28

passed #66368363
win32_build

00:05:30

manual #66368365
windows allowed to fail manual
win32_vs_build
passed #66368364
win64_build

00:06:17

manual #66368366
windows allowed to fail manual
win64_vs_build