Create branch shortcut and dialog improvements

Added a keyboard shortcut (Ctrl+Shift+B) for launching the Create Branch dialog.

Behaviour

The shortcut is meant to create a branch from the current head. This is in opposition to the behaviour of the context menu, which creates a branch from the selected commit. To make this difference clear to the user, I have updated the dialog to indicate the current branch point.

The radio buttons will be selected based on whether the dialog was launched by the keyboard shortcut or the context menu. The branch name and commit indicators also a tooltip, which shows the commit message.

In case where the current head is detached, the dialog will display the first 8 characters of the commit ID.

Reasoning for key combination

Ctrl+Shift+B has been chosen in anticipation of upcoming shortcuts, such as Fetch, Push, Pull, etc. Since Fetch would have to be Ctrl+Shift+F, it seems to make sense to group all the related actions under Ctrl+Shift modifier.