UI Code Cleanup

The code for the user interface has grown over the years - time for a cleanup! We should:

• Check if we can have an own "style" (by introducing an internal module holding all components).
• Introduce a "desktop" mode. The user should be able to switch the mode in the settings; the default value should be derived from the platform we are on (i.e. Android+iOS will use mobile mode, everything else desktop mode for now). Based on this setting, we can adjust spaces automatically, switch between different ways on how to select texts, show certain additional user interface controls and so forth.
• Adjust our color scheme. We use a lot of automatically composed colors. Maybe it would be worth using e.g. some standard colors (as defined in the Pantone palette) instead - especially for the dark color theme. -> Follow up: #460
• Clean up the menu menu structure. We currently put a lot of stuff in the tool bar and - if that is full - in the dots-menu. It would be worth checking if we can improve here to avoid an icon overflow. -> Follow up: #461
• Migrate to another icon font or switch to use real icons instead. Font Awesome is - especially in the newer versions - a bit hard to use in non web use cases.
• Clean up QML code (esp. break down larger files into smaller components and check if we can reuse stuff).
• Introduce (configurable) shortcuts? -> Follow up: #459