Add a API to add/remove items to the "Tools" menu

[ghost]

Make possible add and remove items to the "Tools" menu.
The API is based on my initial proposal but i changed some things during the implementation:

• I removed the has_plugin_menu_item(I don't added it since other functions don't have it's has_* counterpart).
• All function were renamed to use *_tool_* instead of *_plugin_*, IMO this way it's clear that the functions are related to the "Tools" menu.
• The int ID parameter of add_tool_menu_item was changed to Variant ud, used for the callback function.
• The callback format now is callback(Variant ud), which made the code simpler and more flexible.
• The name format for the items, were changed from "my_category/my_item"/"MyCategory/MyItem" to ANY string_Format/My Item Name where the first slice is the item category, this way the name parameter is friendlier to translation(e.g. "category/"+TTR("My Item Name") or vformat("categoty/%s", TTR("My Item Name"))).

Closes #1429.

[akien-mga]

There are now merge conflicts, could you have a look @TheHX?

[ghost]

Sorry for the delay, rebased to current master.

[akien-mga]

Sorry, merge conflicts again. I promise I'll have a look at it for merging then, nobody expressed any concern about the added feature.

Also note that your commits are not linked to your GH account, you might want to fix that too (or not, it's up to you).

[akien-mga]

Ping ;)

[akien-mga]

Rebased and merged it myself in e427ece. Please check it still works as intended in the current master branch :)

Edit: Wait, something went wrong.

[akien-mga]

Alright, fixed for real with b24b52d.