Add window management

[b-coimbra]

Browsers such as Vivaldi allow windows to be split with multiple tabs.
Add such functionality in Vimch with the same keybindings as Vim, eg:

• C-w v to vertically split the current tab with the adjacent one in a new window
• C-w c to remove the current tab from the window split
• C-w q to close the entire window
• C-w > move active tab to the right
• C-w < move active tab to the left

Note: This will conflict with chrome's close tab keybinding (C-w)