[48] Add text editor to VisualGit

[rocketBANG]

Closes #48 Includes some style fixes for the diff panel Moves the save and cancel buttons to the edit panel

Includes a very basic text editing that is accessible from clicking on the "edit" button when viewing the diff of a file Can either save the edited text, or cancel and return to the diff

Test

Setup

Open a repository with 1 or more changes Go to main view (in screenshots)

Test 1 Diff panel opens first

• Click on a file on the file panel
• Ensure the diff panel shows
• Click edit file
• Ensure the edit panel shows

Test 2 Edit cancels

• Open the edit panel
• Edit the file
• Click cancel
• Ensure no changes happened to the file

Test 3 Edit saves

• Open the edit panel
• Edit the file
• Click save
• Ensure the changes were saved to the file

[rocketBANG]

Nice work.

Can you change the green to one that's easier to look at? Maybe use GitHub's green background (#e6ffed) and black text?

I'm not going to fix the colours - that can be done in a separate PR