[Not tested with Python 2.7] [AddonManager] Support for UTF8 characters in macros

[0penBrain]

• Branch rebased on latest master git pull --rebase upstream master
• Unit tests confirmed to pass by running ./bin/FreeCAD --run-test 0
• Commit message is well-written
• Commit message includes issue #<id> or fixes #<id> where <id> is the associated MantisBT issue id if one exists

Add support for macros with UTF8 encoded characters by always writing macro code in an UTF8 file.
Also fix a bug in Execute function

I can't test with Python 2.7. Best if one can do it before merging.

[yorikvanhavre]

Would be better to test with py2 before merging then... Otherwise it can cause some headache later on if users complain

[0penBrain]

@yorikvanhavre : I made a quick test based on my 0.18.4 that uses Python 2.7. It returns error on macros using Unicode characters, but this doesn't come from the modification. Actually, the PR doesn't solve UTF8 problem with Python 2.7 but it doesn't make the thing worse than currently. ;)

[yorikvanhavre]

Okay let's merge then... We'll solve further if people complain