Refactor task widget

[Kasper24]

• Converted the global show signal into proper methods, show/toggle/hide
• No enable function, calling bling.widget.task_widget will return an object with the above methods
• Added the abillity to position the widget differently each time that it is showed. This with the change listed below allows it to align to the currently hovered task widget instead of having a static positon.
• Added the option to provide a wibox, widget and offset arguements to the show method, allowing to the align the task preview to another widget (possibly the hovered task widget).
  This is done by the following code:

task_preview:show(client, {wibox = awful.screen.focused().wibar, widget = task_widget, offset = { y = 100}})

Minor changes:

• Removed the _fn from the placement arguement to keep it consistent with other awesome wiboxes
• Removed the 'widget' prefix from the arguement provided to the object.

Those 2 aren't that important and can be undone though.

[javacafe01]

@Kasper24 Looks good! I still have no Linux machine (laptop will come in a few days), so once I get it I'll test your PR and give it the thumbs up. Or, we can have @Nooo37 test it as well and merge.

[javacafe01]

@Kasper24 Works fine for me, just needs docs.

[javacafe01]

Might want to keep this for a few months so we can notify the user that this signal is deprecated.

[javacafe01]

Merged into #99