A shortcut fix, addition, errors when saving, and dragging pages

Here are a bunch of unrelated changes!

1. Apparently passing a C++ QKeySequence to a QML item expecting a keysequence not always works.
   • When translated, anything that is shown differently on screen (like "Ctrl" -> "Control") doesn't show up or work, so pass it as a PortableText formatted QString, like Elisa does.
   • Make it so clearing a shortcut works immediately (code assumed it was already empty...).
2. • Exit the about page by pressing Escape, because the other 2 windows on that menu do that.
   • Move pages up and down with shortcuts (same as moving lines in KDevelop)
3. Attend to the code checker warnings (this doesn't do anything)
4. When saving a file to disk, a success message was always shown if a save was attempted. Now show a new error message when an added check fails.
5. Do not add the TIFF format to the saving dialog if it is not supported (it was always added).
6. (Re)implement dragging in left panel. This once worked, I tested right before 973a3930 (where a 'doc' reference got invalid), but it didn't look good.
   • Refactor the list delegate so all the visible part is dragged.
   • Use Control instead of Rectangle as the base component to have the left panel minimum width be the widest visible delegate (instead of calculating the total width by hand).

All the changes but the last one are small, but tell me if I should split something into another MR.