Draft: Redesign welcome screen

Current welcome screen doesn't clearly indicate the default template (A4 in mm), gives the wrong message to user and leads to a tedious process every time a document is created (6 steps to change units and size). This PR intends to fix this by creating a New Document dialog which lets you pick document size and units.

TODO:

• Single Create button.
• Let you modify width and height on-screen.
• Create document from right bar, not from the templates.
• Show recent files preview with squared icons
• Remove document units from selection bar.
• Nitpicky "Show this every time" change