Module RefsView: reorganize menu actions

[antis81]

In the past we had a single context menu, that got applied to every RefItem - independent of it's type. At that time we only differed between a "Reference" and "Pseudo-Item" (aka header/folder).

Nowadays, we have more types available, (e.g. a Remote item). This requires us to split the menu according to the item type. Finally, this allows us to show the "right action at the right place".

• Provide menus for the item types Branch, Remote and Tag.
• Provide menu for the item type RemoteBranch.
  • Has the item type Branch. Didn't check yet, but a "sub-type" should be available.
• Provide merge places for fetch & push services.
• Currently, the "action context" in a "checkout" action is set to a "Branch" item type. This leads to crash (assertion).
  • The checkout operation has to be created according to the item type. This also is subject to change, as we plan to replace Git::CheckoutOperation with an RM::CheckoutService. If done right, we should be able to simply provide the item->object() to RepoMan.
• Provide toolbar actions to hide/show the item types (Branches, Remotes, Tags)
  • Use icons according to the item type instead of text.
  • Display toolbar buttons as a "group" -> The buttons should not have a space between them.