D10512: try to preload certain applets in a smart way

[Marco Martin]

mart created this revision.
mart added a reviewer: Plasma.
Restricted Application added projects: Plasma, Frameworks.
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mart requested review of this revision.

REVISION SUMMARY
  preload popups of some applets after init in the background
  based on a value of X-Plasma-PreloadWeight in the desktop file
  if present, otherwise some default values based on the applet
  type (Provides)
  
  Save the weight in the config, if an applet is never opened,
  slowly decrease the weight, when it reaches 0 don't preload it
  next start, increase every time it gets opened, so at the moment
  it's quite aggressive about preloading, in order to not do it
  a lot of plasma startups without touching the applet are needed
  
  Applet with a very big weigth will be preloaded immediately,
  therefore having an impact on the time it will take to have
  a panel visible and usable, while lesser weigths will preload
  after a random number of seconds between 2 and 10, so will load
  in the background after everything is started

TEST PLAN
  Plasma starts up correctly, applets load correctly and can be added
  correctly both those expanded or collapsed.
  plasmashell appears correctly usable without too big hiccups even
  while it's loading things in the background
  
  some numbers:
  without preloading, plasma takes around 64 mb of memory after startup
  when preloading everything about 94, so it's a cost of about 30 mb
  which is not negligible.
  don't have precise timing, but if everything gets preloaded immediately,
  the time to get an usable desktop appears to be at least doubled,
  while the delayed preloading (except just a couple of applets) doesn't
  seem to have a big impact on the time needed to get an usable desktop

REPOSITORY
  R242 Plasma Framework (Library)

BRANCH
  mart/preload

REVISION DETAIL
  https://phabricator.kde.org/D10512

AFFECTED FILES
  src/plasma/data/servicetypes/plasma-applet.desktop
  src/plasmaquick/appletquickitem.cpp
  src/plasmaquick/private/appletquickitem_p.h

To: mart, #plasma
Cc: plasma-devel, #frameworks, michaelh, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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