[context] Refactor activity handling.
[context] Add set-default action to context changes.

Activity groups can follow sink state and do actions based on the sink
state. When activity group is left (that is, bthsp changes to ihf for
instance), "default state" determines how the active or inactive actions
are performed. Context-rules may contain set-default action, which
changes the "default state" value for given activity group.

There are three different default states:

active   When activity group changes active action is executed.
inactive When activity group changes inactive action is executed.
state    When activity group changes active or inactive action is executed
         based on followed sink state (if sink is running or suspended,
         active or inactive, respectively).

Configuration example:

[activity]
device       = bthsp
sink-name    = equals:sink.primary
active       = sink-name@equals:"foo",property:"bar",value@constant:"true"
inactive     = sink-name@equals:"foo",property:"bar",value@constant:"false"

[context-rule]
variable     = call
value        = equals:active
set-default  = bthsp:active

[context-rule]
variable     = call
value        = equals:active
set-default  = bthsp:state
Merge pull request #16 from jusa/activity-default

Modify activity group default state dynamically.