UI overhaul

[someretical]

Is your feature request related to a problem? Please describe.
There are some parts of the UI that I don't feel satisfied with right now:

• The [x] button should just close the application outright instead of going back to the main menu
• There is no toolbar at the top (as is standard)
• There is no status bar at the bottom

Describe the solution you'd like

• Use QStackedWidget to manage a "stack" of screens so it is easy to revert back to the previous screen. This is for situations when I want to create a dialogue window, but want it to appear in the main window instead.
• Call setEnabled(false) and setEnabled(true) on the main window itself to disable all input.
• Use the status bar to indicate actions like hashing the password, importing diary, saving entry etc.
• Create a permanent worker thread to delegate file system IO/hashing etc.
• Use QToolbox instead of QTabWidget as QToolbox supports icons.
• Create a wizard for creating a new diary.
• Make the diary entry editor its own entire widget that takes up the entire screen.
• Use QListView for displaying diary entries (instead of HTML).
• Populate the toolbar with actions currently available.
• Allow increasing/decreasing font in the diary entry editor widget.
• Allow opening multiple diaries at the same time
  • The tabs of a tab widget can be hidden or shown
  • Would require a more comprehensive diary class
  • Still only one network manager is required

Other things

• Use a python file to build the whole project. So the only thing needed to do on GitHub actions is to install the required python packages and then run the python file.

Guidelines

• Try and not use fixed size policies.
• Use appropriate Qt UI elements even if the learning curve is high (cough cough the whole HTML rendering fiasco)
• Use Qt provided APIs for JSON, strings etc. That is, only fall back to the C++ Standard Library if needed.

Final remarks
These are pretty major changes so it is expected that only AppImage'a for Ubuntu 20.04 will be built during development. macOS and Windows are a whole other nightmare to deal with.

[someretical]

For possible colours, see https://github.com/nealmckee/penumbra