BranchActions dialog

[jeremypw]

Fixes #1577

An attempt to reduce the click requirement of common branch actions like checkout and to make it more keyboard friendly. Potentially could be more discoverable by linking to a button in the UI.

The existing hot-key <Ctrl>B now raises a dialog for all branch actions (some not yet implemented).

• Implement checkout branch (local)
• Implement checkout branch (remote)
• Implement Create New Branch
• Show recently used branches

Other branch functions can be added later such as Delete, Commit, Push etc

The list of branches includes categories "Recent", "Local" and "Remote" in alphabetical order and is filtered on names containing the entry text.

The "Checkout" action is the first page in a stack linked to a StackSidebar.

"Recent" branches only contains branches checked out in the current session of Code but may be persisted later.

[jeremypw]

@danirabbit This is still under construction but before I do too much on it I would welcome your opinion on whether this is viable. It doesn't have to be a dialog of course - it could be a pane or a stack for example and accessed by a button rather than a context menu. The main idea is to have all the git branch actions in one place, accessed by one hot-key, sharing code where possible.

[jeremypw]

I have dog-fooded this a while and found it very useful even with just a checkout functionality so lets get a review.

[jeremypw]

Need to remove/hide unimplemented functions.