Shortcut configuration

[letsfindaway]

This PR supersedes #686, which again is based on #460.

[letsfindaway]

@kaamui: what do you think about providing this PR without the configuration dialog? It contains some remarkable improvements for shortcut handling anyway:

• Shortcuts for switching tools are active even when the tools palette is hidden. Currently the shortcuts no longer work when the palette is hidden using the top left button.
• You can temporarily switch tools by pressing and holding the keys. Consider you are drawing something. When now pressing and holding the shortcut keys for the eraser, you can switch to the eraser. When you then release the keys, you are back to the previous tool. This function might also be useful for the laser pointer.
• You can switch tools while drawing. You can e.g. start drawing using the pen, then while the mouse button is still down, you can press the shortcut for the line tool and continue with a straight line etc.
• I think also the option to use the shortcuts without the Ctrl modifier is quite useful.

[kaamui]

Hi @letsfindaway,

I think the configuration dialog need improvement, so I agree we could benefit the improvements you mentioned in the next release and separate it from the configuration dialog planned for 1.8. I have some feedback to give you, I simply didn't find the time to write them down.

Go for the two separate PRs !