Restructure .gitignore file
gitignore: Restructure existing rules

Organize the rules into groups corresponding to the tools creating
these files.

Use blank lines and comments for clarity.
gitignore: Keep README and COPYING files
gitignore: Additional cmake rules
gitignore: Merge rules from po/**/.gitignore files
Remove unnecessary translation scripts

The new gitignore rules will automatically ignore these.
gitignore: Merge dozens of "Thumbs.db" rules

It suffices to apply the rule once in the root directory.
gitignore: Reduce number of false positives

Especially in utils/umc_dev/ a massive number of files matched ignore
rules. As tracked files can't be ignored, this had no real consequences.
gitignore: Fix order of rules for Makefiles

The exceptions were applied before the generic rule, thus they had
no effect.
gitignore: Block some extensions
gitignore: Merge rules from projectfiles/
gitignore: Include more rules from subdirectories

That is: data/lua/, scons/, and utils/
gitignore: Merge rules from doc/ and data/tools
Remove remove-potcdate.sin from wescamp-import

If we delete the remove-potcdate.sin scripts, we may as well remove the paragraph importing them.
Merge pull request #199 from aquileia/ignore

Restructure and merge .gitignore files