Automatically set "Billable" checked in some cases

[jeroenheijmans]

It might be good to have the "Billable" checkbox ticked by default for certain customers/activities.

E.g. for "generic overhead" it might be non-ticked, but for "CustomerX / Development" it makes sense to have it ticked by default.

[FreekPaans]

Actually, the current business rule is that only :hourly tasks can be billed, so we should only show the checkbox when it's an :hourly task and indeed then tick it by default.