ENH: sync with upstream linux support

[klauer]

This should bring qtpydocking up-to-date with ADS.

Changes:

• Brings better (but experimental) Linux support
• Adds default Linux stylesheet
• Miscellaneous cleanups (unused imports) and minor refactors

Miscellaneous notes:

• Not yet interactively tested on Linux ( cough I set LINUX=True on OSX...)
• At some point, I'd like to move to using Python properties much more to lessen code duplication
• I tried to split this PR up into matching commits with the upstream library, but it ended up being too much work. Oh well...

[klauer]

TODO: add the new linux stylesheet to the MANIFEST as in #8

[klauer]

• I don't plan to continue this project
• Merging this last PR, then archiving