add: button to terminate massdash

[singjc]

Description

Added exit button to close and terminate massdash process. This closes the opened massdash tab in the browser, and terminates the PID process of massdash.

This adds an additional dependency: pyautogui, for closing the tab.

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

GUI

Test Configuration:

• Firmware version: popOS
• Hardware: system76

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

  Contents (#128)

Other

• //github.com/Roestlab/massdash into gui/exit_app
• button to terminate massdash
• docstring info
• pyautogui dependency to project.toml and setup.cfg
• //github.com/Roestlab/massdash into gui/exit_app
• execption handling for shutting massdashdown
• shutting app down on windows

[singjc]

@irahorecka can you test this branch on macOS, just press the Exit button to see if it closes the tab and kills the process properly? I don't know if the pyautogui.hotkey('ctrl', 'w') will automatically translate the 'ctrl' to 'cmd' in Mac.