Rename pkexec policy file to .policy extension.

Rename com.github.calamares.calamares.bin to
com.github.calamares.calamares.policy. This matches how other policy
files in the same directory are named.
CMakeColors.cmake: Use colors only if stderr is an interactive terminal.

In particular, do not write ugly escape sequences to log files.

Fixes #124.
Missing define, check_chroot_call replaced

not really needed here, subprocess.call well tested
add the define
Merge pull request #136 from kkofler/pkexec-policy-file-name

Rename pkexec policy file to .policy extension.
Merge pull request #137 from kkofler/fix-issue-124

CMakeColors.cmake: Use colors only if stderr is an interactive terminal.
Automatic merge of Transifex translations
Fix displaymanager sddm config call.