Implement Convert To in path commands

[MewPurPur]

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• Adds a boot splash.
• Splits off Utils.popup_under_control_centered() from Utils.popup_under_control().
• Implements the ability to convert path commands.
• Fixes drawing of T path commands if the last command that isn't a T wasn't Q.
• Fixes mistakes in which commands are allowed or disallowed in the Insert After dialog for path commands.
• Improves the logic for exporting SVGs as PNGs.
• Tweaks the logic for drawing the graphic, should be 50% faster in most cases. Partly addresses an idea in GodSVG Optimization roadmap #212.
• Fixes the graphic in the export SVG dialog from being able to go of the checkerboard background.
• Improves the graphic display in the export and import SVG dialogs and makes it faster to load.
• Fixes focus indication on context menu buttons.