Replace the continuous_mode checkbox with a radiobutton group

Replace the continuous_mode checkbox with a radiobutton group

The wording of the continuous mode checkbox confused 100% of respondents
in an unscientific poll, and no one (including me) could come up with
better wording.

Instead, take a suggestion from Robin Luo, and try a radiobutton group
where the choices reflect the observed effect (continuous_mode off =
"Lower memory usage", continuous_mode on = "Faster browser startup
time"), not the implementation.