App changes

• Fullscreen mode: just press F11 to toggle this mode.
• New entity tile display modes: entity tile can now be displayed in more ways: Repeat (ie. repeat tile to cover entity bounds), Cover (ie. covers the full entity bounds), Fit Inside (stretched to fit inside entity bounds).
• Removed deprecated “Crop” from Entity tile render modes
• Added an option in LDtk settings to start the app in fullscreen mode
• Fixed cursor position while zooming
• Fixed “close” button in Rules panel
• Fixed scrollbar in Help panel
• Updated README