Consolidate table header actions code, allow filtering of actions

[DD32]

The attached patch consolidates all the <select name="action"> code into a function print_table_actions() which also allows the filtering of the actions.

My primary motivation was due to me personally not liking the dropdown lists, esp. when theres usually only one or 2 actions in the table. Allowing filtering allows a plugin to step in and replace it with buttons (Which i've also attached)

Plugins which add actions, will have to hook in on load-$pagenow and perform their actions there i believe.

[DD32]

Plugin which replaces selects with buttons

[DD32]

Theres probably a better way to deal with some of the params of the function really. I somewhat doubt its needed to include them all, but i'm unsure

[DD32]

patch refresh.

[DD32]

(Patch will be refreshed upon request, With a better streamlined arg list if anyone actually goes for it.)

[ryan]

Postponing for consideration after 2.7.

[Denis-de-Bernardy]

patch needs a refresh. some core dev feedback would be sweet here.