Allow plugin to work for gedit < 3.14, and fix detection of 4-space indent.

Needs testing on gedit 3.14 which i don't have available right now, but it should work.

Tested on gedit 3.10.4.

Indent width detection was improved by considering indents that increase by one indent-width from the previous line to have more weight. This should give better detection in all cases, and fixes detection of tests/spaces-4.c which was being detected as 2-space indent.