Improve mirror tool

[rubengc]

Hi guys

On my last proyect I found some problems with mirror tool that led me to disable it, I will point everything:

• Allow pair mirror: I try to make objects of 20x20, the problem about that is mirror tool only let you make a 19x19, because only let you do odd voxels objects. Will be awesome change it
• Do not apply mirror on layer change: I work with this objects of 20x20, I have disabled mirror, after open other project I enabled the mirror tool, and again I reopen the project with 20x20 and mirror is enabled by default, I do not notice this, and when I change to another layer the mirror is applied with wrong result

I recommend a mirror tool like Space Engineers (yes, a game) you could see this tool here:
https://www.youtube.com/watch?v=uDllYDRBMXE

My recomendations about that are:

• A semitransparent wall (with same axys colors, like SE)
• Allow change mirror axis and mode hitting again the mirror tool key