S#10387 - Rework pluginlib actions

S#10387 - Rework pluginlib actions

It changes pluginlib actions to contain only a single and simple context (and
other one for remote directional buttons),
consisting of 7(9) buttons: up/down/left/right, select OR short select and long
select, exit and cancel (plus 2 for scrollwheel targets).
This ensures contexts don't clash with other contexts and simplifies them, at
the expense of reduced versatility. However, the versatility made it largely unusable
due to the great number of targets.
This should allow for using pluginlib actions safely for the most simple plugins (e.g. almost all demos).

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@26202 a1c6a512-1295-4272-9138-f99709370657