Refactor "gui" package dir structure to match "tui" dir structure

[reaganlo]

The "gui" package has some sub packages that are leading to duplicate code. Remove the ambiguous sub packages and make the directory structure of the "gui" package similar to the "tui" package.

This will make the code more clean and concise especially before we add more pages to the GUI to make it feature parity with TUI.

• Remove the sub package "pages" and move its contents within the main "gui" package.
• Rename the "init.go" file to "common.go" similar to that in "tui".
• Remove the sub package "common" and move its contents to the "common.go" file.