do not enable experimental feature by default

[r10s]

“Sync All Accounts In Background” resp. its disabling became an experimental feature, which is okay in general.

however, experimental features should not be enabled by default.

the other way round, disabling all experimental features should result in sth. as “standard state”.

so, as the option as such should stay experimental, it needs to be be sth. as ~~“Only Sync Foreground Account”~~ “Only Sync Selected Account” (tooltip could read "Synced when selected" then)

EDIT: the following is offtopic and not part of the issue: ~~nb: also the “Power Icon” shown on the account icons is a bit weird. what about showing a question mark intead? this would also make it clearer to the user what the impact of the option is:
not knowing if there is sth. new
(a “?” is shown where otherwise a counter appears)~~