replace tab stops with spaces and remove trailing whitespace characters

[jpue]

This PR tries to standardise the appearance of code across different editors, development environments and/or platforms by enforcing an indentation of four spaces instead of (one) tab stop(s).
Moreover, this PR cleans up the code a little more by removing trailing whitespace characters.

This PR deliberately leaves some files untouched to avoid unwanted changes (learning from the mistakes I made in the “const PR” some weeks ago):

• Markdown files (since they use trailing spaces to separate paragraphs)
• most files under debian and resources/schemas as well as many files under platforms and resources (I am not familiar with the syntax/formatting rules for these special files)
• some translation files and some old Japanese documentation (resources/docs/html_ja_JP) (so as not to break encoding stuff)
• the C++ code in plugins/hid and plugins/peperoni, whose indentation is mostly based on tabs
  (Replacing all of them would have cluttered this PR and made it very confusing to review.)

[coveralls]

coverage: 35.037%. remained the same — jpue:whitespace into mcallegari:master