Pressing F temporarily hides on-canvas overlays

This MR implements one of the most requested features in Inkscape: the ability to temporarily hide all visual clutter such as selection frames, on-canvas manipulation handles, grids and guides.

The rationale is that an artist often wants a quick, uncluttered preview of their artwork without having to lose the existing selection or UI state. This is the Fast Preview introduced here.

After the change, pressing F temporarily hides all "overlays": grids, guides, selection cues along bounding boxes of selected items and on-canvas controls such as the manipulation handles of the Selector or Node tools. The overlays remain hidden for as long as the key is down and reappear as soon as it's released. It's still possible to use tool-specific shortcuts; e.g., with the Selector tool, you can still use arrows or Shift+Arrows to move the selection around.

At the moment, the key used for this purpose is hardcoded at F but new infrastructure to enable customizability in the future is currently being planned and should arrive before 1.3 release.

Demonstration video

hideui

Known issues

• If you hold down F and then click on an element other than the canvas, e.g. a dialog, then the key release event may be lost and the canvas overlays will get stuck at invisible. Pressing and releasing F again with canvas in focus restores everything to the working order. Coincidentally, the quick zoom Q key suffers from the same issue. Both will hopefully be fixed by the upcoming new infrastructure, which will also make them customizable.

Fixes inbox#1253 (closed)