Release v0.4.0

• Support dropping two files anywhere on the main window to diff them
• Same for the "Select files..." dialog
• Add "Switch files" (ALT+TAB) menu to switch left <-> right
• Add "Open left/right file/loaction" menus, because double-clicking and CTRL + double-clicking is not that discoverable
• CTRL + double-click into a pane to open its file's location
• Add a Help window describing the non-obvious features
• Show correctly proportioned horizontal scrollbars
• Scroll views when mouse-wheeling over the left pane, too
• Nicer separator between left and right pane
• Improved color handling, esp for dark themes
• Save window size and position, cascade new windows
• Updated translations, thanks all translators!