Add Panning and Zooming

[mattrobineau]

Description

This PR introduces changes to allow a user to pan and zoom the canvas in Satty and closes #93.

Changes made:

• Adds zooming functionality
  • Adds two toolbar buttons for zoom in and zoom out
  • Adds scrollwheel up/down zooming
• Adds panning functionality
  • Holding and dragging middle mouse button will pan the image

Notes

• The scroll event gtk::EventControllerScroll needed to be added to the main window otherwise the event would only fire when scrolling on the toolbar.
• handle_zoom zooms by an arbitrary value (+-0.1) chosen solely because I thought it was a good value. It might be worth considering allowing a user to modify this value in config.
• Zooming does not currently support zooming based on the cursors location, it will always zoom from (0,0).

[gabm]

I just tried your PR and the functionality works as advertised, great 👍

For the current feature-set I'd like to suggest the following changes

zoom

• I think the center of zooming should be:
  • a) the center of the screen, if invoked by the toolbar buttons
  • b) the position of the mouse pointer, if invoked by the scroll wheel
• to change the center of the zooming operation, you need to translate the chosen center to the left-top corner, then zoom and then translate back

pan

• the pan speed should be equal to the mouse speed
• on my screen it seems that the canvas moves "slower" than the mouse moves
• that might have something to do with the scale factor (?)

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Usability and UI

• we need to discuss whether we want toolbar buttons, an extra toolbar or none
• we may want to respond to the arrow buttons for panning
• we may want to respond to the zooming gesture for zooming
• I think we want a shortcut to reset all zoom and translation
• we need to think how the background looks like - black or white (or any other color)
• we do want to export/save the "smallest possible canvas", that still covers the image and all drawings. That allows users to draw outside the image
• I didn't look at the code yet 🙈